^ Since the Magisters Sidereal were the high priests/priestesses of the Old Gods, I honestly expect to see at least one come into the new game. It's very much unclear how many are alive or not besides Corpheus, since you can spare the Architect and the eaten magister could possibly be alive. One showing up is quite possible. As I've mentioned in previous discussions, I'm hoping for a possible ally/companion option against Solas, though that's probably unlikely. And the existence of the Disciples make things interesting for the Grey Wardens, as well as the arrival of two Magister Sidereal and the rumored civil war they're facing.
To add to previous statements, the Forgotten Ones were enemies of the Evanuris, in a similar way to the Titans of the Ancient Greek religion were the enemies of the Oylmpian gods, or, in a sense, it's like the Jotuns and the gods of Asgard in the Norse belief. However, as the potential evil gods, there really isn't much known about them.
Also to point out, there are some similarities in the stories of the Forgotten Ones and of the Old Gods of the Tevinter Imperium, which could be a factor in your confusion. It can be difficult if you're new, so don't worry.
And I do agree that we need to see more of them in the next game.