I was actually thinking the same thing. There have been five blights. We've seen Mythal and Fen'Harel. 9-5-2=2. Maybe one is Sandal or a third party who will be introduced in the next game. That leaves one Old God soul for the next protagonist. The only problem with this is that making the protagonist an Old God would kind of screw up the more personal, relationship-based part of the game. If you are an Old God, you will likely be far less approachable than previous protagonists. The Warden was a hero saving the world, but so were their companions. Hawke was a just a poor refugee when they met most of their companions. The Inquisitor's power didn't matter too much because their companions were mostly also in positions of power. If the protagonist is an Old God, it will be very hard to make a sensible portrayal of their companions being as close to them.
There's also the huge issue of power as far as gameplay. "I don't need a party! I can turn people to stone with my eyes!" This could be countered by making the protagonist "weak after their slumber," I suppose, but I kind of think that would feel like a writeoff. Their gender could also be a potential problem, as, so far, we have only seen Old Gods as people of their own gender. It seems like Solas is actually a special case, as, based on what he says in trespasser, he seems to be the original Fen'Harel, body and all. He says that Fen'Harel is a name that he took "as a badge of pride", though, so it is possible that he named himself after the god, which would mean that the real Fen'Harel could still be out there somewhere, leaving another Old God soul somewhere. I don't know. I'm probably overthinking it.