• The Four Symbols: Byakko, Seiryu, Suzaku and Genbu. Seyryu would obvious be the more important one considering what we're dealing with.
  • The Other Aesir: Thor and Loki, Not likely since Odin has his own in universe lore that differs from the Norse, but still interesting.
  • Olympians: I can see Aphrodite being a sort of "love" primal, twisting the concept around and using it to feed off the aether. The group in general can be seen as "evil counterparts" to the Twelve or even the Twelve themselves corrupted as a result of a botched summoning.
  • Yiazmat: From XII we have a superboss that could take weeks to kill (maybe hours at best), therefore why not escalate his badassery to that of a being that is second only to BAHAMUT
  • Hell Wyrm: Also form XII and a dark counterpart as Yiazmat