It won't be Vhala, she would be in Combat since she's a party member. If she joins, another slot will be added to Combat.
Elleani and Simon have a pretty developed relationship already. She'll have a slot. She won't be in Leaders because she's uninterested in politics, she's just forced to participate right now.
Ivala? Her RP could be used for something else. Right now the Ivala rp score is mostly about some high pitched thief, as far as we know the name of the variable could be a red herring. Of course there's also a clear possibility that she'll join.
Also there's the Goddess of Magic, who had a cute interaction with Simon but not much else. She may still get some on-the-fly RP calculations, à la Tertia.
As much as I'd like Mestan to join as is, I think his RP will get transferred to someone else. Like he'll fuse with Xestris or something. (It's just an example.) Anyway, I think we get too much RP calculations for him for it to be irrelevant harem-wise, so Mestan or a character linked to him will get a third slot. Maybe even Mithyn since they have similar names and she doesn't have RP yet.
Vunne have some RP too, she might join. I don't think that's likely since her RP follow the same model as Vhala's, but that was also true for Sabitha, so heh.
I don't think Xestris will join. She'll be happy ever after with the Anak, or die, one of the two.
Who am I kidding? Last spot is clearly for Grubbak. :p