I'm hoping that the Inquisitor will randomly run into Hawke's love interest as they are looking for Hawke. It'd be nice to see that they're ya know, concerned about Hawke's disappearence, and also it'd be a great chance for some banter with Varric if he is in the party. (And if Cassandra is there she could ask the LI about some event that Varric told her about in da2 and the LI can say 'what, that never happened?" prompting more fun Varric exaggerating the truth banter with her...)
Beyond that:
-Sebastian: He threatens to invade Kirkwall if you don't kill Anders. I'd like to see him follow through on that in some form in the next game, or he had tried to follow through only for it to end disasterously. I just don't want Sebastians MASSIVE threat at the end of the game to go completely ignored.
-Aveline: She is either Guard Captain of Kirkwall or she may have returned to Fereldan and become an officer in the army. Both outcomes could provide a decent enough reason to interact with her.
-Hawke's sibling if they are a Warden. If not...no thanks.
-The Friends of Red Jenny. I want answers.
-Feynriel-given that he can interact with the Fade in unique ways he seems like a potentially important asset if you kept him around.
-Warden/Hawke...I mainly want confirmation that they are alive, and that Hawke will get the chance to reunite with their love and go on with their life in (relative) peace. If they show up, I imagine they will be wearing helmets...anything else is just...too messy. If we do see the Warden I imagine it'll be with a group of other wardens (maybe some you know like Sigrun, Carver, or Alistair), and you see your warden off in the distance while you talk to one of the former companions who off handedly mentions that the Warden-Commander you see 'over there' is also the Hero of Fereldan.