I don't think Briala will be a third party in the Orlesian civil war. Yeah, she does say that the city elves will have to fight and win their freedom, but I don't see her actually be thematically opposed to either Celene nor Gaspard. Neither of them really defend the Orlesian ingrown racism. Celene will fight for the empire first, yes. Gaspard don't particularly care for the elves, no. But they're not opposed to the elves as such.
What Briala will have to fight is not a war for the throne, or for land, but for rights. That's a whole different fight and she'll no doubt have to fight a few nobles. But I don't see conquests of cities or armies remotely figure in (especially since the elves would be crushed and Briala knows this). It's social upheaval, but not outright civil war stuff.
So unless Briala starts attacking Gaspard or Celene I don't see her being part of that plot. I do however think she'll be in a plot, though not neccessarily in person.
I do concur about the mysterious figure in the ending though. That was no doubt our villain or it's Dragon (not a literal dragon, the archetype).
I think the dalish will have a plot, a dalish protag would be rather wasted if we didn't, but like the city elf fight I do not see it being part of the civil war plot. To it's benefit, since it won't have to share spotlight.
As for Fen'Harel... he'll only play part metaphorically.