What universes do you think can merge really well? Where the events and characters of one universe could easily be seen occurring in the same universe as another setting?

For example, there are a couple that I can see easily existing alongside the Dresden Files:

Stargate. Admittedly, this is mainly due to Todeswind's excellent work with his Endless Pantheon series. Having the Goa'uld not be false gods but former gods makes a lot of sense, making the Tok'ra's slow assassinations of System Lords be due to having to keep Outsiders imprisoned puts a spin on why they are so frustrated with the SGC, linking the never seen in-universe Furlings with the Sidhe as part of the ancient alliance, combining the concept of the Garden of Eden with the Ancients and Atlantis... there are just so many points where Todeswind has had the two series converge that I just feel in awe of how well they mesh together.

Supernatural. Specifically, the early seasons. There being a number of mortals who have had their lives turned upside down by the supernatural and decide to go cross-country killing them off makes a lot of sense and works as a nice backdrop to the setting.

(Aspects of) DC/Vertigo Comics. I'm talking about the Endless, the Green and Red elemental embodiments of plant and animal life, Constantine, etc. It feels like a lot of the supernatural stuff in Dresden Files that exists alongside reality doesn't really get touched on apart from the Fae. The Courts get larger and larger presences in the books as the series goes on, with the only other competitors for screentime being mostly physical beings like vampires. Adding more layers to reality feels like it could expand the series a bit.

The X-Files. Partially. I haven't seen a lot of the series, mostly some episodes here and there. But an FBI agent looking into supernatural events while groups like the Venatori Umbrorum try to keep it secret makes for a decent story.

 

Dresden Files would work amazingly merged/fused with the 1974 show Kolchak the Night Stalker starring Darren McGavin.

The show is about Chicago newspaper reporter Carl Kolchak investigating mysterious crimes, all of which were either supernatural or sci-fi (mostly supernatural). It's literally a perfect fit. You could easily have Kolchak be the founder of Arcane which Susan worked for after inevitably being fired from the paper he worked for.

 

It's kind of obvious that there are as many that can mix as that can't, which is to say, numberless, but as a general rule, I like something between 'these two universes are basically the same' like cop show A and cop show B and 'these two universes are incompatible'- one person literally messaged me on ffn asking if I could help write a crossover between Mahoromatic and Ed Edd n' Eddy and I was at a stage in life where it felt more appropriate to express my confusion rather than just ignore it. I want to see some clever stitching that makes the two universes work together, and I'm willing to suspend my disbelief about pointless details if the story's good enough. I've never seen it done before, but I'd really like to have Harry Potter in a crossover with Call of Cthulhu.

 

Harry Potter works with:

- Mary Poppins
- Narnia
- The Night Circus
- Paddington
- The Sword and the Stone

Dresden works with:

- Buffy
- Underworld
- Scholomance
- Artemis Fowl?

Red Rising works with:

- Battlestar Galactica
- The Exapnse
- Firefly

A Song of Ice and Fire works with:

- Buffy
- Certain X-Men (e.g. Wolverine)
- Star Wars (stranded Jedi without lightsaber)

 

Are we talking show or books-verse Expanse?

As a books superfan, I have a hard time reconciling the two, although I do like RR.

 

There's different sorts of setting-wide crossovers, if we're going beyond a very simple "character falls through dimensions" to introduce Sam Winchester into Harry Potter.

The first is a blending. The two settings are blurred together, with aspects from both coming into play. Generally, this would require a bit of thought on how to make it so that everything fits, rather than just swapping around character names, although that's also possible. As an example, the "Harry Potter the Avatar" story that was started a while back. Hogwarts is set up as a home for benders, magic has been replaced by bending, and it's (almost) a unique setting.

The second is chimera. The two settings exist (and always have existed) alongside each other, with it just being co-incidence that they've not interacted before. This is easier to do with 'real world' scenarios, like Brooklyn 99 and House. Technically, there's nothing to say that these shows *don't* exist in the same continuity, and given that they're completely different cities, you can very easily have Peralta drop off a very ill suspect for House's care.
A big help for this is if the settings have 'the masquerade'. So long as both elements of the crossover exist outside the public eye, you can get away with saying they both exist at the same time - see "Harry Potter the Denarian". I'm not saying this is the *best* example, but it's one that a lot of people here would be familiar with. Both magical societies are hidden from the vanilla mortals, and don't interact with each other at all.

Spoiler

A third option, that of the 'graft', would be where you take a few things from one setting ad add them to the other. Such as "Wolverine in Lord of the Rings", or "Harry Potter except I've SIed in Lockhart".

For a 'chimera', both stories have to be in a similar setting - you couldn't do Teletubbies and EastEnders, because there's not that much empty green space in London. You could *possibly* do it with Harry Potter, and have them be magical creatures living on a preserve, but it requires a bit more effort. But something like "In the Name of the Wind" and "The Witcher", which both have a unique setting, couldn't be an easy chimera. You could ISOT the setting, or do a simple blend and just add the Witcher schools dotted around the map anyway though, and it might effectively be the same.

However, this isn't taking 'tone' into account at all.

 

Well... there's crossovers and then there's crossovers.
If what you mean by "merge" is a 'fusion,' that can be difficult to do, because you are trying to make two separate worlds into one, cohesive, internally consistent, world.

Fusions are also more prone to being completely lame, unless handled exceptionally carefully. Want to make me hit the X, fast? Make Hogwarts a school on Coruscant or in Westeros.

Perhaps contravening expectation, mixing two settings with magic is harder than one setting with magic and one without, because you have to try and merge two sets of metaphysical rules.

For instance, Harry Potter and Anita Blake: Vampire Hunter are not an easy fit because... the metaphysical 'rules' of how werewolves work are so different; 'muggles' know magic exists, even if they don't know much; and using harmful magic on people is pretty much an instant death sentence, at least in the U.S.. The way vampires work in HP is pretty vague, though, so that's not so bad. It's when you get into explaining why the preternatural world (vamps, weres, etc.) doesn't know about the magical world (if, in fact, they don't) that things get even stickier.

A Harry Potter/Bell, Book and Candle crossover would be weird, because you'd have to explain why the witches and warlocks of the latter can't cry or blush, and lose their powers if they fall in love, while none of those apply to Harry Potter's witches and wizards. Also, Bell, Book and Candle's witches don't seem to ever use/need wands, with everything being done with gestures and words, potions, or with the aid of a familiar... though you don't see them slinging fireballs, either.

On the other hand, merging Harry Potter with Law & Order, C.S.I., or Family Ties is a snap.

Crossovers where the two different fictional settings are treated as different time periods in the same world, which one character or group of characters time travels between, can be somewhat easier to manage, if the metaphysical rules governing those worlds aren't too different.

Crossovers where the two different fictional settings are treated as separate dimensions/realities, which one character or group of characters travels between, are much easier to manage.
Harry trips through the Veil and ends up in the Marvel Cinematic Universe. Okay. *SHRUG* Not much to finagle into shape with that one, beyond one thing...

Power levels.

Putting Optimus Prime at the Battle of Hastings would be funny for a Curbstomp Fic, but the hurt he'd dish out in a serious fic would remove most of the mystery about who's winning that fight (Hint: Whichever side Optimus Prime is on).
There's a reason that, half the time Harry ends up in Middle Earth, it's without his wand.
There's also a reason that, half the time Harry ends up in Middle Earth, it's as an elfling who bends over for a bunch of adult men, but that's because there aren't enough firing squads.

The more fantastical, comedic, or 'just for fun' your crossover is, the easier it is to ignore inconsistencies between the two fictional settings.

Harry Potter/Virtually Any modern day, completely mundane, non-magic setting
(Law & Order, Columbo, Leave It to Beaver, E.R., that older show also called E.R., Knight Rider, It's Always Sunny, Charlie's Angels, Father Dowling Mysteries, The A-Team, are you getting the picture yet?)
Fusion: Absotively. Same Reality, Different Eras: Posilutely. Dimension Travel: Indubitably.

Harry Potter/Tomb Raider
Fusion: Doable. Dimension Travel: Double doable.

Harry Potter/The Incredibles
Fusion: Why not? Dimension Travel: Sure thing.

Harry Potter/Alvin and The Chipmunks
Fusion/Dimension Travel: Totally not throwing shade, but this is a cursed setting. Whether it's the version with human hybrid chipmunks or the actual-size chipmunks there are real questions to be raised about why they aren't in a lab being studied, and the backstory almost certainly involves shit that would get Aberforth Dumbledore arrested. I mean, if they aren't the offspring of someone stuck halfway in their animagus form, then what the hell happened here? And why do Jeanette Miller, a Chipette, and TMNT's Irma Langenstein share the same outfit?

Speaking of...
Harry Potter/Teenage Mutant Ninja Turtles
Fusion: Awesome. Same Reality, Different Eras: Radical. Dimension Travel: Totally tubular.

Star Trek/Star Wars
Fusion: HAHAHAHAAAA! Same Reality, Different Eras: No, really, to whom do you owe money? Dimension Travel: Still ouch.

Harry Potter/Star Wars
Fusion: Doable, but lame. Same Reality, Different Eras: Fine. Dimension Travel: Abso-friggin'-lutely.

Harry Potter/Star Trek
Fusion: If you simply must, but I question your taste. Same Reality, Different Eras: Fine, I guess. Dimension Travel: Sure, sure...

Harry Potter/D&D
Fusion: Which setting? Same Reality, Different Eras: You fuckin' wot, mate? Dimension Travel: Which setting?

Borderlands Series/Tron
Fusion/Same Reality, Different Eras: Absolutely! Borderlands is literally just the ultimate outcome of the Tron movies. What happens in Tron if not the digistructing of people and things? Dimension Travel: Unnecessary, but sure, fine.

Harry Potter/Twilight
Fusion: Again, HP vamps aren't described all that much, but werewolves? Lucy, you got some 'splainin' to do. Dimension Travel: Fine, just understand that any notable HP wand-waver could absolutely go sickhouse on any vamp/were that doesn't get the drop on them. A Creevey dropped in Forks could become king of the world, if he played his cards right.

Harry Potter/Back to The Future
Fusion: So you're telling me technology can alter time, but magic can't? Dimension Travel: Same shit, different 'verse.

Harry Potter/Bayonetta
Fusion/Same Reality, Different Eras: So there are these witches, but they're not like your witches, and there's a Heaven and Hell, but they're not like your afterlife... It's fuckin' complicated. Dimension Travel: I mean, sure, I guess.

Harry Potter/Westworld
Fusion/Same Reality, Different Eras/Dimension Travel: Sure.

Harry Potter/Half Life Tri Duology:
Fusion/Same Reality, Different Eras/Dimension Travel: The right man in the wrong place could make all the difference in the world.

Harry Potter/Mike Tyson Mysteries
Fusion/Dimension Travel: I- I guess... I mean- What?

Harry Potter/Wheel of Time:
Fusion/Same Reality, Different Eras: Ages come and pass, but the magic system is supposed to stay the same, yo. Dimension Travel: Alternate realities are already present in WoT, but I suppose there could be ones where HP style magic is a thing? You'll probably get killed the second you cast in front of people who can't feel you channeling, and your wand is undoubtedly going to get taken by anyone with half a brain. "I've become an expert at casting two spells without a wand: The Killing Curse, and a Switching Spell that I use to swap stomach acid with the contents of this ever-full canteen of Forkroot Tea."

Any non-TV media/Westphall Universe
Now you're just looking for trouble.

ProTip: Never, ever, ever, have Harry end up in another setting because of a dropped/exploded potion, unless the story ends with him waking up on the floor/in the hospital wing and finding out it was all a dream/hallucination/MDMA trip.

Pointless Trivia: If it was specifically mentioned as a crossover with Harry Potter in this post, I've probably written at least a few words of it.

Edit: lol, I see Thaumologist basically covered what I did, during the eight hours between when I started this post, stopped to play OW and make FO4 textures, then came back and finished the darn post.

 

I most definitely meant fusion when I talked about merging. For a long time, whenever I saw stories with Multi-Xover tags, I avoided them. In my mind, it meant multiple dimension hopping or characters from many different series being brought into one. But I stumbled across a couple stories that took the true fusion route--merging together many disparate series into a near-seamless whole. Which got me thinking about other potential fusions and thus this thread.

Are there any enjoyable stories that you have come across that fit this description?

 

I've forgotten more of these stories than I can count (because I've forgotten them, obviously), so something more in line with what you're looking for may come to me later.

It's not deadly serious, as far as being an intricate melding of several different worlds into a very detailed and complex whole is concerned, but if you want to see a multi-crossover that touches lots of different properties... It's a bit out there, and it has (as far as I know) been abandoned, with its last update having been almost 11 years ago, but I thought it was a lot of fun, and was very disappointed it never finished.

Harry Johnson and The Headmaster's Socks
Harry Johnson and The Lunatic Scientist
Harry Johnson and The Deathtrap Girl