1) It's going to have to be some killer app to get me to buy another iPhone. I won't even update iOS because I want to keep my jailbreak.
I don't use my phone for apps (I own quite a few internet capable computers). I use it for calls, texts and emails when out so this killer app is going to have to be so spectacular it changes the way I use my phone. That hasn't changed in 10 years so Apple's got some serious work to do there.
I'd be much more likely to come back to Apple if they started allowing me to customize my phone like jailbreaking does. But they won't so unless they shock me with this killer app it isn't going to happen.
2) I doubt the SE is coming back. It's a nice phone and I'd love to see it in the size of the plus models. It's proof that Apple can make a camera flush with the casing if they want to. But I don't see it happening.
3) I don't care about, really, because when I buy a phone it's going to get used for about 3.5 years. I tend to buy the largest size flagship offered with the most capacity in the color I want. If Apple offers a phone I like, I'm willing to pay the price for that. But that hasn't happened since the iPhone 5. Of course, paying less for what I want would be great though.