Could the next Prius be sexy?
Lol, no.
...it'll likely get the valvematic thing from the Corolla LE Eco, which will increase the highway fuel rating. I'd wager there could be something new about battery placement or even battery type, leaving the NiMH to the basic 'c' and using LiIon batteries for the regular liftback and PHEV (as is currently the case for the PHEV).
That, and perhaps some drop in weight?
It'll likely loose a bit of the wedge shape, but not much. I'd expect the funky split rear-glass will remain.