Split View and True Multitasking
This is unlikely, but I’d like to see some form of Split View on iPhone, at least on the Max models. I’d also like to see draggable app windows on iPad. Apple is really making the push for the iPad to be your next “computer,” but without some form of true multitasking, that’s hard to do.
Another welcome improvement would be background multitasking, currently closing out of an app halts many of its features. Could this have a large impact on battery life? Yes. But it doesn’t matter how much battery you have if you can’t use your phone in the way you want to use it.