Arcanists only have one dual-faction Master (Mei Feng), and I don't see Colette or Kaeris as being anything but standard arcanist. Thus, I would expect the last Master to be dual faction something or another.
Guild and Resser slots are full, which leaves one Neverborn, one Outcast, two Gremlin, and one Ten Thunder slot open.
There's already an Arcanist/Ten Thunder master so I think we can reasonably rule that out. So that leaves us with two Gremlin slots, one Outcast slot, and one Neverborn slot.
If Lucius is indeed Guild/Neverborn as many people suspect, then that lowers the other open slots down to just Outcast and Gremlins. That would point towards the final Arcanist master being either Arcanist/Outcast or Arcanist/Gremlin, both of which would be interesting and would bring plenty of new options to the faction.
Personally, my money is on Arcanist/Gremlin, because it just seems amusing as hell. Maybe Wong? He did show up in the Gremlin fiction that dealt with uniting all of the Gremlin masters...