Here's the full list of the remaining summons that appeared in other FF but did not make an appearance in any form in the XIV yet

Asura [FFIV]
Remora [FFV]
Crusader [FFVI]
Maduin [FFVI]
Raiden [FFVI]
Seraph [FFVI]
Tritoch/Valigarmanda [FFVI]
Quetzalli/Palidor [FFVI]
Ragnarok [FFVI]
Hades [FFVII]
Brothers [FFVIII]
Doomtrain [FFVIII]
Eden [FFVIII]
Quezacolt [FFVIII]
Pandemona [FFVIII]
Madeen [FFIX]
Valefor [FFX]
Anima [FFX]
Magus Sisters [FFX]
Mateus [FFXII]
Adrammelech [FFXII]
Zalera [FFXII]
Shemhazai [FFXII]
Hashmal [FFXII]
Zeromus [FFXII]
Exodus [FFXII]
Famfrit [FFXII]
Chaos [FFXII]
Ultima [FFXII]

All of these only appeared in a single main FF so at that point wouldn't be suprising if they ended showing up as simple bosses.
I'd see the next trials being Byakko, Genbu, Suzaku and Soryu. just because they appear on the map around kugane