Maractus. When I caught it I liked it, just cause it has that kind of "I'm high" face. It has such great abilities too. I would use it, but it's so frail and slow. It should get an evolution that's a lot faster and has some more bulk, and maybe a bit more to its attacking stats. I have no idea what it would look like though.
I can see a Pike-like evolution for Basculin.
An evolution for Emolga would be nice as well… though all the Pikachu-like electric types could use it.
Stunfisk could use a less derpish evolution with better stats.
Finally I think the elemental monkeys could use an evolution; just to improve their base stats (and maybe their movepools).
Exactly how is Stunfisk anything like Magikarp? With the right training, Stunfisk can be an absolute beast. Unless you're basing this on appearance, in which case it looks absolutely nothing like Magikarp in the slightest.
Personally, I'd like to have seen Stunfisk and Durant get evolutions. They are both already pretty good in terms of stats, so I imagine evolutions would be even more beastly.