Emulators are what I miss from android. GBA has one of the best libraries of any console, plus there’s the custom rom scene. And I have this absurdly powerful phone that can’t run any of it.
I think emulators are likely at the top. I can see Ryujinx absolutely being a top download. Big enough that Nintendo would absolutely arm up their legal team. Imagine every iPhone and iPad becoming a Switch!
Then region-locked apps. There are tons of games and apps in general that change from region to region. Chinese games are never like their NA counterparts. This would effectively unlock every region to the user.
Lastly, just random shit. It would be cool to install 3rd party apps. FOSS apps. Just anything that doesn't need to go through all the red tape of the App Store.
Just the option would be nice. If iOS was closer to being as customizable as android and I could get an iPhone halfway decent in the latest Gen for under $500, I might consider switching. The walled garden has always kept me out.