Honestly, the processor is fine with me. Since it's an S model, we won't see any physical differences and thats were I think the 6 design could most benefit from. Spec wise, just give it 2 or 3 gigs of RAM and call it a day. Other than that I'd be happy.
As for the next phone, I don't mind if they keep this design, but honestly if I could change the 6+, I'd make it a tad thicker (easier to hold) while allowing a bigger battery, shorten the top and bottom bezels and trim the side bezels. Call it a day. Oh and that pesky RAM bump.