We know Pentos is OOH Targaryen character and Army.

Braavos is OOH Stark and Lady.

Norvos is OOH Martell and Lord.

The ones that have not been spoiled yet are OOH Lannister and OOH Baratheon. I'm sure those will reduce another common trait like Ally, Asshai, Knight, House Tyrell, King, Queen, Bastard, Clansman, Maester, etc. Considering Volantis is NOT reducing some common trait(not common in Greyjoy at all at least), I wouldn't put it past the designers to choose something odd like Traitor or Fool.

I would put my money on Knight and Ally considering the abundance of them and that it wouldn't make much sense to me if it were other traits. :-/