This is the perennial problem for writers - too many ideas.
I have so many fighting for space in my head that often I can't focus...
So this is what I do.
I take a few of the most promising and start to plan. This will tell me if they have legs or are just what I call 'twinkles' - bright shiny stories that have no substance.
I jot down as much of the story line as I can think of. I add in notes on my main characters, setting, themes etc.
If I can't get above two pages - it's a twinkle.
Say I can get a few pages, I now decide upon the structure that would best suit it.
This is the single most important decision.
How you tell your story ie the methodology, will make or break it.
If I can't settle on a structure, I put the project to one side. It may be more than a twinkle, and I may come back to it, but it still has no legs.
If I do alight upon the perfect structure for the story - I go for it.
The result is a lot of pages of twinkles - but I would rather that than spend months on a project that ultimately goes no where.