We’re starting to run out of impactful old bosses and summons to turn into raids and trials now, I wouldn’t be surprised if the next 24 or even 8 man raids ended up being somewhat original, like the Void Ark series.

That said, possible options include:
FFVII Weapons (possibly trials like the warring triad, or 8 man raids like Alexander, fighting mini bosses along their slumbering bodies for fights 1-3 until they wake up and we fight the whole thing in fight 4)
Jenova (could go anywhere really but probably not trials, but with the whole biological weapon thing Garlemald has going on I wouldn’t be surprised at raids)
Eden (unsure how it would fit into lore, but it would be similar to Alexander)
Four Fiends, FFI or FFIV (could easily be trials or 8-man raids)
Emperor Mateus (could go a similar route to Cloud of Darkness in the Crystal Tower series)
Valefore as a new beast tribe primal
Magus Sisters as potential raid or trial primals (could be the ‘fairy’ beast tribe, or Garlean magitek themed)
Asura
Zona Seeker
Valigarmanda
Madeen
Then we could potentially get some Bahamut revival in maybe 6.0 or 7.0 with Neo Bahamut and Bahamut Zero.

But I doubt they’ll all get made into significant fights, lots of summons have ended up being minor mobs or dungeon bosses.