Honestly, I'd just port the Digivolution mechanic from Survive (and thus, the anime) to the Story Games. It's kind of ridiculous how Digivolution works in those games, having to do it as a side activity feels too inspired from Shin Megami Tensei/Persona. I'd rather undertake quests to unlock specific, branching evolutions for my Digimon partner(s) and then evolve them in battle.
Digimon
For fans of the Digimon franchise from Bandai.
4 is also my least favorite, and yeah, the digivolution mechanics are very lackluster in that game, but really, I've never been fully satisfied by the digivolution mechanics in any game.
1's training-based evolution lines and 3's ability for any starter to evolve into all available evolutions in the game were great, but I want even more. I want every evolution to feel like it's been specifically tailored to the unique relationship I've developed with my digimon, potentially even generating all-new digimon within the game on the fly.
Obviously, game development isn't really capable of doing something like that right now, and even if it was, digimon isn't a big enough franchise to be able to afford such a huge undertaking, but it's always been my dream game.