Not exactly a pokémon, but a way to use them: I enjoyed using Nidoking, Rhydon and possibly Golem in Red/Blue and giving them off-type moves in addition to earthquake ou rock slide. These had the widest coverage in my team. Would have been fun to be able to use them like that in go (and no, there is no way I buy my Rhyperior a 2nd charge move and burn through TMs until I get Surf, this is just not worth it in GO).
I’d like Ditto to be more useful too. I sometimes dump one in a gym, thinking a 400cp mon has some staying power, while being able to transform into a 2000cp mon for fighting! Unfortumately it’s so frail it’s useless. Same thing on attack.
Overall, I got the impression you could use almost any pokémon of a given type and that was good enough. I could use Hypno as my main psychic fighter, Pharamp (edit: I meant Ampharos) as electric or Tentacruel as water, that was fine. Not in go, where so many pokémon are useless.