My forecast
1) Faster, lighter (for the 4 inch), brighter (they haven't done brighter lately). Better quality camera (with more MPs).
2) Faster WiFi and cellular (that'll be improved somehow, maybe not major).
3) A 128GB model, we're due, right?
4) New form factor. They've changed every two years for a while now.
4a) Larger is likely.
4b) I'm not so sure about the curved and I'm not sure I want one!
One and two happen almost every model. It's about time for them to bump up the size, but that's just speculation. Items one through four I'd have "predicted" without ever hearing any rumors.
I don't need secrets or access to the supply chain, most of this is just common sense at this point.
Would not "predict" 4a and 4b; I might guess on a larger screen at this point but it would never cross my mind that they'd curve it.
But then again, I'm one of the holdouts that never thought they'd call the iPhone 4s the 4s.
Gary