Yeah it's honestly, again, an underrated game for anyone looking for a "Pokémon but not Pokémon" fix.
Nibbles would honestly not be my pick for FF, because I'd prefer a character from the best-remembered first game. Rosie I would like better, but wouldn't she technically be able to work as an alt for the main protag?
On that note, if they went with the FF1 hero, would you be imagining that as a prop/summoner type character? Or a Pokemon Trainer type concept?
If they could make the size work then the T-Rex would be my choice; it's the most iconic imagery in the series IMO and I think fans would love the concept (sorry
but you can't hold a candle to this).