[@BluBlood] So, the way I see it, we are at a crossroads here and it can go either way:

If Nadine wants Gabriel more than his submission and is willing to give herself up to his control as well, they can be romantically involved as we were contemplating. This, of course, leaves some room for frustration to build up but those could directly feed into the idea of them using their powers for sinister purposes and looking for people to toy around with, an overpressure valve for her sadistic tendencies and his domination to keep the relationship intact - maybe they share, maybe Nadine plays with the other while Gabriel focuses his domination entirely on Nadine, maybe they change things around every now and then... no idea, but generally speaking, Gabriel would probably not give her THAT much more than what he's already offered

The other option is that she refuses. They could still be friends but I think this would leave a little bit of a sour taste and it could lead to, for example, Gabriel rather quickly looking into ways and means of forcing Nadine into submission and/or Nadine doing the same for Gabriel.

My vote would be for option one, since I'd rather they be allies and lovers who do increasingly horrible things together than them being enemies in disguise, but you're the one who knows whether Nadine is up for it, not me.