It's not how I'd do a HL crossover but we certainly need more of them.

 

Not exactly a crossover per say, but a Harry Potter story with the Inside Out concept might be cool. Especially if it explores how occlumency/legilimency works. And maybe even have Voldemort's horcrux as a major character, like Joy or Disgust.

 

I think earlier in this thread someone said Harry Potter crossover with Warhammer 40k.

Wouldn't it be more appropriate to make a crossover with Warhammer Fantasy instead? I mean, it's still grimdark, just more appropriate and heck, you can just transport him in either Brettonia or the Empire, hell try Albion, there isn't much on that place.

 

Harry Potter/How to train your dragon

A Harry Potter from a world where the fight with Voldemort or the hunt for his most devote followers is still going on washes up on the shores of an island of the barbaric archipelago. No wand, no clothes, nothing.

He can still change into his animagus figure (because it's cool and gives him mobility and an edge), which means there should be magic. Getting very desperate to find his way back he decides to get a wand, even if he has to make one himself. As he is about to kill a dragon for its heartstring Hiccup and Toothless show up and attack him and even force him to retreat.

An increasingly desperate Harry preying on dragons and the vikings hunting him.

Long-term you could even think about crossovers into Frozen (nod to Republic) or Tangled.

 

If you allow Apparition without a wand, then it's an insta-win for Harry.

If you don't, then he still may have Legilimency and other useful tricks, but he's at a serious disadvantage.

 

I would rule Apparition out, because like you said it's overpowered.

I would give him an avian animagus to grant him mobility between the islands and fledgling Legilimency to sense intent or detect lies. Harry would be 18 or 19, a young adult forced to grow up rather fast.