Banette. So much Banette. If I could pick one thing to have in this next gen, it would be an evolution for Banette. I love its design, but it's just so bad.
Weezing could really use another evolution too. For that matter, so could some other two stage poison types (Muk, Swalot, Garbodor, Arbok), but I don't care about them as much.
Skuntank is another one that's pretty bad (not as good as Weezing and better than Banette) that I'd like to see become more usable. I like the design and it's got a good typing, but its stats are just crappy.