Furret comes to mind. It's essentially a worse Raticate, and that's saying a lot, so an evolution would do this long boy some justice, I feel. While we're at it, let's make other Johto mons less awful, like Ledian, Ariados, and Sunflora by giving them evolutions. Other Pokémon that feel "incomplete" to me, like Delcatty and Kricketune thanks to their awful stats, could use evolutions as well. I'd also give Tropius one in this regard, although making it an even bigger dinosaur seems a little too much to me.
On the other hand, though, I'm not totally on board with giving evolutions to "gimmicky" Pokémon like Smeargle or Wobbuffet. Smeargle's gimmick is that it can learn almost any move, so if you were to give Smeargle an evolution, with (I'm assuming) higher stats across the board, its gimmick would be even more obnoxious to deal with. Wobbuffet's a similar story, and if you made this thing even bulkier (by either evolution or the Eviolite Wobbuffet would benefit from thanks to it), its gimmick of hitting back twice as hard as it took would make it much more annoying.
Chatot definitely needs an evolution, though. Give us more parrot Pokémon, Game Freak! Who's to say Castform's creators can't give it an evolution, either?