Fulminato i don't think ivala will join. she will be an ally like vhala. xestris i think is a safe bet. in the hall of carnality and replay sphere there are a few simon/xestris sprite actions. locked for now.
zelica is still a unknow factor. can join, or remain an ally (or she cannot surviving xerces procedure)
If Ivala stays an ally, and as an unkown either dies or doesn't join the Harem, who could the last one be? To me it seems unlikely that an entirely new character will be introduced to fill that final slot.
On a side note, at the waiting screen when checking relationship points it provides a value for Reletima.... So is that just Sierra messing with us or is she a stand in for someone else?