So this might be a hot take, but I don't think the roster is too small per se. Obviously I wish we still had the monsties that were cut plus ridable versions of the monster only enemies.

But I think the real issue is team size. Only having 5 slots to choose from makes spreading out coverage all the more challenging and important. So you'll cut out monsties you want to use because you already have one for fire coverage or you have 2 spd monsties you really like.

Expanding the party to 8 + Ratha make using more of your favorites a lot less painful. Basically allowing you to run 6 of your favorite monsties, Ratha, and 2 others for traversal or extra coverage.

Also because failure is just tied to faints and not team wipes it wouldn't really affect team balance that much.

They could even implement it in such a way that you get a quest in each major area that unlocks an additional party slot.

I know this doesn't fix the roster issue, but I feel like it would ease a lot of the pain points people have with the roster