Like many said, this series is far from over.
Solas will likely be the next villain but there are still far too many loose ends to account for:
-Two more Old Gods left to cause blights and the Warden's are on the verge of collapse.
-Five or six Darkspawn Magisters left besides Corypheus depending on wheter you spared the Architect or if he'll stay dead.
-Seven Evanuris left besides Solas and Mythal who will each likely be a major antagonist.
-Titans who have just awakened to wage war on all factions.
-That thing worse than blights implied to happen when all Old Gods die.
-What's in the Black City and who, if he exists, is the Maker.
-Lack of a definitive protagonist to conclude the series.
In my opinion, none of the DAI games we played so far had any conclusive ends because they each set up the settings for the next one and we haven't even gotten into the real main conflict yet.
Originally, Bioware intended for Hawke to become the main protagonist of all following DA games like Shepard but because his game bombed they backpedalled on the idea and switched back to multiracial characters in Inquisition.
I get the feeling that they didn't give up on the concept so much as pushed it back so that the definitive Choosen One could have a proper introduction and impact on the story in a way Hawke failed to achieve.
The way Inquisition ends feels like you've been told the story of Admiral Anderson, you stepped into the shoes and lived the life of the big good whose actions put the Alliance at the forefront of galactic conflict but who ultimately just set the stage for the young messiah to bring everlasting change.
The Inquisitor and his inner circle are the only sane people not blinded by bureocracy, corruption or bigotry who can move a sizeable force against Solas when he finally makes his move in much the same way that Anderson and Hackett were the only sane people who could give Shepard the armies he needed to save the galaxy in the nick of time.
In that same vein, one could say that Solas is going to be the Saren of the true DA trilogy. He is that well-intentioned extremist who believes that salvation comes at the cost of destruction but ultimately serves as just a pawn for the sealed evil.
The Evanuris are pretty much the Reapers, unfathomable ancient enemies who could easily destroy the world and whom only the protagonist can challenge and survive,
In turn, the Maker is something like the Catalyst, a bastard who never spoke or manifested itself until the very end but was in fact manipulating everything from behind the scenes.