hi!!! i got a 4 for AP 2D, i definitely recommend something personal that you can explore through your art, it can be as niche or broad as possible.
and also don't be upset if you want to change it at anypoint!! ur themes develop, i started off with "childhood memories" and that ended up turning into "exploring the inner child" aka, my inner child
There are tons of videos on youtube and stuff that you could look at. But i think you should try to find something personal, or something you can physically get too. like dude, RESEARCH RESEARCH RESEARCH!!
an idea could range from "romantisizing a mundane life" or "image household appliances as bug-sized"
it doesnt really matter WHAT you do, just as long as your writing is strong and ties closely with your art.
The whole point of AP Art is that you are EXPLORING an idea with your art, you're trying to EXPRESS an idea
Experimentation (Not Progress) Photos are really good to fill in some of those gaps and allow closer looks into your idea
Here is my AP portfolio if you wanna look at it for a loose example :3
AP Portfolio Slideshow
don't get caught up on being cliche,
as long as you incorporate your own interpretations and experiences it will be okay, and will most likely evolve into something unique!