who's Adrian? 
A character from the Asunder novel, that most people hate for what she ends up doing, but who I found likeable and ultimately a rather tragic figure.
Origins: Sten and Shale.
I know that Sten's kinda busy these days, but it'd be fun to see him once more. And Shale is always a blast.
Awakening: Velanna and Sigrun.
I could see Velanna probably having some interesting dialogue with an Dalish Inquisitor or about the Elven uprising in Orlais. Sigrun because there's already a woeful lack of female dwarves in the series.
Leliana's Song: Sketch and Silas.
Both characters I enjoyed, though I imagine they'd probably have ended up on opposite sides of the Mage-Templar conflict. Silas wasn't really fleshed out that much due to being a last minute replacement for Tug, so I wondered where his story would have gone? And did Sketch ever manage to outrun all those assassins after him?
Witch Hunt: Finn and Ariane.
Okay, basically both new companions from Witch Hunt, but Finn is utterly adorkable and would geek out over that major event that happened in The Masked Empire, while Ariane was one of the few Dalish characters who doesn't have any major problems with humans (that we saw anyway). It'd be fun to see them again.
(Ah heck, maybe the Dog as well? Sure if Mabari age like regular hounds he's going to be seriously old by now, but I don't doubt Dog would still be a Badass Grandpa even now)
DA2: Merrill and Isabela.
Merrill and Isabela are some of my favourite companions in the game, I'm fond of Isabela's romance arc in particular and how much depth it gives to the character, while Merrill and her obsession with the Eluvian was a tragic storyline. It'd also be interesting to see if Hawke managed to break her obsession with the Eluvian in DA2, only for her to slip back into old habits due to the events of Masked Empire.