Doomtrain's been teased for a while. Could see it being one of the new raids, though not sure whether it would be the 8-Man or the 24-Man.

Rhalgr seems a distinct possibility too. With how the resistance tried to summon him once before already, I could see someone making the leap and going full Thordan/Shiva with it in Stormblood. Probably Griffon. Who is totally not Ilberd.

Other than that, I think that it would be pretty cool to see Hades.