Yeah, I'll agree with @Zeelthor. Neville as BWL has been done a billion times (at least 7000) at this point. That isn't necessarily a problem either. Neville as the BWL gives you a lot of freedom to take Harry in a different direction.

If you want some actionable feedback, you need to start figuring out what the consequences are of Neville not being the BWL first and then bring that back to this thread. Is Harry still the one who needs to defeat Voldemort? Does he help Neville? And like Zeelthor said, you can let that consequence ripple. That's where you need to differentiate your story. As it stands, neither your post nor your summary shed any light on what you want to write, so I'm not sure where any of us can help.

 

Summary =/= Premise and Concept.

Let's break down what you actually have from the summary:

• Neville as BWL
• Slytherin!Harry

Those are really the only two ideas that stand out to me. The rest is a mixture of generic looming threats and information that's already obvious (that this is an AU need not be stated most of the time, at best just say AU and be done with it). As a result, there's really not much to talk about here.

If you want some actual feedback, you will have to give us something to work with.

For example: Harry is in Slytherin. So what? How does that change canon? Does Voldemort still go after the Stone in Year 1 and the Snake in Year 2? Do the adventures change? Who are Harry's friends in Slytherin and why them? How is this Harry different from canon? Is Harry meant to join Voldemort, or is he still anti-Voldemort? Maybe he's neutral and is forced to pick a side? What does Harry want to achieve? What are his goals, dreams, aspirations?

 

I agree with everyone above.

Here's one possible way to make your story stand-out, though it's a bit of a cheat. Give Harry some unique ability that he hasn't got in canon and use that to make your story different. James and Lily are alive so perhaps Harry obtains said ability somehow during a childhood mishap, or maybe he's just born with it and that's why Voldemort picked Neville instead of Harry if it's some kind of handicap.

Example: Harry is a seer and gets flashes of the future. But he's not great at interpreting them and often if he tries to he ends up screwing up what he saw so that it gets worse instead of better, or he assumes something will happen and then it doesn't because he didn't try to make it occur, etc.

Example2: Harry was assumed a squib at birth due to a rare magical illness (leading to LV not targeting him), but treatment in some cool plot-relevant area as a child fixed the issue. However you can have Harry be influenced by wherever that place was in whatever way you like to give him both a new weakness/issue and an ability.

Example3: James and Lily raised Harry but died about a month before Harry started at Hogwarts. So Harry is dealing with everything that comes from that, including trying to sort out guardians, deal with grief, etc. Perhaps James could tell him some Potter!secret on his deathbed that Harry then investigates or perhaps the new guardian he gets is Madeye Moody, who knows?

...and so on. These are lazy ways to make your story standout and frankly I don't like the ideas I just wrote that much (okay I kind of like the third one). But I hope it illustrates the idea that unless you are capable of writing the BEST Neville!BWL & Slytherin!Harry story out there (you probably aren't) you need to do something else to make people remember your story as opposed to the thousands of other ones with similar concepts.

Any idea (even those above) can be done well in the right hands. Pick an idea to make your story standout and then let things ripple out from it. It can be a choice as others mentioned above or it can be an AU element as I mentioned.

You've likely got some scenes in mind for later in the story that stand out in your head. Are there any early changes you could make to the basic story to make those scenes better, make them hit harder, or make them more interesting? If so, then walk those things back to what needs to happen in Chapter 1 and let it help set your story apart.

If someone is doing a story search for a Slytherin!Harry or Neville!BWL story then they will have trouble finding the specific one they're looking for. But if they ask for the Slytherin!Harry story where Harry spent time in Africa, or the Neville!BWL story where Neville had a twin, or the Slytherin!Harry story where Harry could eat memories, or whatever... people can identify those stories quickly because of the unique aspect they had. ((I made those up))