Sableye should have gotten one. With no weaknesses, an evolution with better stats would easily make it a good OU contender. Also, I think Luvdisc could have used one. I mean, seriously, it's just an awful Pokemon...
Since this hasn't been closed or moved, I'll throw my 2 cents in.
I would like to see any pokemon that hasn't evolved to get an evolution or a pre-evolution. Any pokemon that already has 1 evolution, I'd like to see a 2nd evolution.
If not for another evolutionary stage, I'd like another evolutionary option. I like the idea of pokemon being able to go down different evolutionary paths (Kirlia and Gallade/Gardevoir or Snorunt Glalie/Froslass etc.)
I always thought i would be cool if there were more eevee evolutions (strictly from a type perspective) or if you could further evolve the current evolutions into something else. Also I think it would be cool if Rapidash had another evolution, like into a pegasus looking creature with wings, that would be cool Maybe Spiritomb could evolve?
Most pokemon don't need evolutions. Not every pokemon needs to evolve and not every fully evolved pokemon needs to be powerful. Unless they can come up with a good evolution they should just leave them alone.