Meowth should return again as a pokeball. Truth is, there's just too many Generation 1 pokemon, sorry Meowth
Sudowoodo could use Mimic and copy an opponents move and fire back at them to give them a taste of their own medicine so to speak
Octillery could use Octozooka and fire ink balls at an opponent while making a smoke effect to prevent anyone from seeing them
Plusle or Minuin would appear by themselves, they would use a weak shock attack, but if both are on screen at the same time, they could KO all enemies on-screen.
Delibird could use Present and toss a present at the nearest opponent. It could either do damage, be a bomb, or recover health for that opponent
Aerodactyl could use Rock Slide. He could fly over the screen dropping rocks on all opponents
Miltank could use Rollout. I think she should be similar to Marill, though, shortly disappearing offscreen.
Smeargle could use his signature Sketch. Whatever move the person who released used, Smeargle would "learn" and attack the opponent with that move several times.
Hariyama could attack an opponent with Smellingsalt
Pineco could surround himself with Spikes, damaging any opponent who touches them