I'd actually counter this statement entirely. It is stated at the beginning of the game that it's largely because of waste and pollution in Reborn City that Pokemon don't want to live there, so if Adrienn cleaned it up then I'm sure there will potentially be some new Pokemon in Reborn.
That being said, I think the bulk of the available Pokemon are going to be available in massive sidequests. I'd predict Oshawott being available in the Coral Ward shack (where Cain got his), Bulbasaur/Snivy exploring the Malchous/Rhodocrine Forests, Shellder/Tentacool/Slowpoke/Frillish/Staryu available via Super Rod (probably in E16, not definite), Pinsir/Heracross (the one you didn't pick) available in Aventurine Forest, as well as Scyther, Azurill, Shroomish, Togepi, Drilbur, Axew, and Vullaby being available in the city somehow. I would also bet on maybe one or two of the pseudo-legendaries to become available, probably Dratini or Larvesta.