Magus Sisters -- Maybe a fit for the second Void Ark raid. Mhachi magical prodigies of some kind.
Dr. Lugae/Lugaeborg -- Might wait until the Garleans become a central player again, because he could be a rogue Magitek researcher.
The Four Fiends -- But to really give them their due, they'd have to be an entire raid series similar to what they're doing with the Warring Triad now.
Zeromus -- Primal material if there ever was one.
Asura - Also Primal material.