you forget that in some strange way after the fist ending marche ritz and cid came back to ivalice because in the first ending you see everyone returned to st ivalice. Wich means that if Marche and Ritz can go back why cant mewt.

Mewt can be a great asset to the story, giving him the task of getting ivalice back together. you can also use doned as a tactician, like him and mewt working together making plans to defend ivalice while march and tirz carry those plans out and ocasinaly throw in there own plans.

this sequel idea can bring such a good story but the idea needs to be thought out more.