the Z was the first proper flagship phone from Sony. Waterproof features and all.
It was the first and there were many lessons learnt from it. ie: flaps, display, bezel etc etc
Thus, come the Z1 which address the issues above (earphones without flaps, magnetic cable charging without opening flap, etc)
Hence, I would think the changes to the new phone itself would be marginal. be it Z2 or Z3 etc.
Even in the looks dept, there wouldnt be much difference because Sony is taking the Omnidesign as the core
The difference would be in the hardware spec IMO
But I seriously hope the camera would be improved cos we have the hardware for it.
I'm still hoping there will be Red for the new phone. I would move if there's Red. if not I might think twice about "upgrading" from Z to Z2