To my mind, any Romulans sympathetic to the Dominion cause probably were so because they empathised with them retaliating against perceived Federation encroachment.
The Romulans admire power, and the Dominion certainly had it, but I can't really see the Romulans happily offering themselves up as anyone's lapdogs.
I think the Breen would always have been on the cards, whenever they made their move, but beyond that I'd have to say it would probably have been one of the lesser seen races, like the Tholians as mentioned above.