There's not really something that would be 'revolutionary'. However, I think there are some things that could 'wow' users:
- A new design (really new, because the current one is basically an iPhone 12. Something like going from a 3GS to 4)
- True variable optical camera zoom, essentially reducing the need for 3 cameras and going back to 2
- Display tech: maybe 'directed HDR'? Imagine having a photo where you look at some water and than when you tilt your head, the way the sun reflects the water also changes. I'm not talking about a software effect here. I'm talking about just moving your head around and you seeing different parts of the image from a different perspective, like in real life.
- A breakthrough in battery life. Like instead of you charging your phone each night, you'll need to charge it only like once every few weeks.
Also one I think would be cool was like a build-in 'privacy screen protector'. My mom had one for a few years. When she got a newer model, she asked me 'where in the settings menu she could enable that' again. She looked a bit surprised when I told her it was not a device feature, but instead something that her previous screen protector did.
I think it would be pretty neat if you could actually artificially reduce the viewing angles. Apple could possibly do it the Apple way as well: when you're using Face ID, or entering a password or something like that, it would just pop on to avoid shoulder surfing.