Yuna from Final Fantasy X is the first summoner character that comes to my mind, of course, i doubt she'll become playable, Since the next FF rep in line is probably Tifa, but based on her Dissidia NT moveset, she could have a cool moveset.
I think a Bravely Default rep would be cool, if the series keep getting new entries i could see it happening at some point, i'm not sure if it will happen next game, i think the next SE reps in line are Geno, Tifa, 2B maybe a Octopath rep (Tressa is my guess) since the two OT games did pretty well for a relatively new IP.
I do think an AT for BD next game is defeinitely possible, like Edea or Agnes poping out and casting spells or performing skills from BD.
Now that SMT has been brought up, do you think the SMT reo would be Nahobino or Demi-Fiend (I never played SMT, but those are the names i heard the most).