Other SE titles to collab with?
Chronotrigger
The Star Ocean series
The Mana Series
The Saga Series
A new collab event from the Dragon Quest Series

Outside SE
PSO2. They got a collab on their side a long time ago.
A new MH collab. Game has no shortage of monsters. Could base it off Rise and have a rampage style encounter.
Super Mario RPG. Though not likely simply because Nintendo generally doesn't like their franchise characters in things outside their own platforms. The title itself was a collab but I think is more Nintendo than Square.
Biomutant. Assist furry creatures, get weapon and armor glamours that look like cobbled together junk. You know you want your warrior hitting things with a toilet on a stick :P