>>52654098First-stage babymons can piggyback on big mons for support. Maybe have the big mon hold an item like a saddle/stroller or not. I'm not sure.
Either way, they can both use a move in a turn (maybe have the babymon be limited to a much lower level or not allowed to use attacking moves so it doesn't have a big offensive output) and share their abilities.
So for example, Bulbasaur can ride on Charizard, making it so the pair has both Chlorophyll and Solar Power. Bulbasaur can use Synthesis, the powder moves, and screens, etc.
Or use Grass Pledge to boost Charizard's Fire Pledge if it can use offensive moves.
Not only would this be interesting for battle, but also for marketing, letting the cute NFE mons be useful.
It could be any pair of mons, or be restricted to specific pairs (such as Charizard + Bulbasaur, Venusaur + Squirtle, and Blastoise + Charmander).