Some good suggestions in this thread. Although I probably would prefer new characters I would like to see Merrill return, as others have said with the mirror and all the other elfy stuff going on I think Merrill would be in her element. I also did really like the Sigrun idea, I really liked her in Awakening and I also love Morinth.
Love the desire demon idea. Would be funny to see Sera's reactions to her. I'm guessing she would be conflicted because on the one hand demon but on the other boobs.
I've also been thinking for a while that a tranquil companion would be interesting. They make me think of artificial intelligence (Data, EDI, the emergency medical hologram, etc) but of course they aren't artificial. They have no programming like an artificial intelligence would which I find interesting, so what motivates them? This is something I'd like to see explored further.
Of course the problem with a tranquil mage companion is that they would no longer have any magic. But I agree with one of the previous posters, there's no reason a tranquil mage couldn't have learnt how to use a bow.
I already had a backstory in my head for a tranquil rogue and circle blood mage so I might as well share.
Tranquil (former mage) Elven Rogue: Raised in Kirkwall's alienage she gained her skills as a rogue from her parents (who made their living as thieves). When she started to show signs of magic her parents arranged for her to be trained by one of the (many) apostates in Kirkwall in order for her to be able to hide her magic from the templars, she still continued her training as a rogue. It wasn't until years later that the templars caught up to her and locked her away in the oppressive Kirkwall circle of magi where she met....
Human Circle Blood Mage: Raised in Kirkwall's cirlce of magi she recalls little of her parents or her life before the circle. A naturally gifted mage, she strives to be the best and spends much of her time researching different types of magic including forbidden magic. When the elven apostate is brought to the circle the two young women become friends.
Tranquil Elven Rogue: Although she missed her life outside the circle she enjoyed being able to practice her magic freely without having to look over her shoulder. She was not as skilled as many of the other mages in the circle, but with some tutoring from her new human friend she passed her harrowing. The two women became inseparable and the elven woman's feelings for her friend grew, but she dared not confess how she felt. When the templars found a book of forbidden magic the human had been reading the young elf confessed in her stead and took the blame to save her. Before being made tranquil she made herself a promise never to tell the templars about her friend's use of blood magic. She kept the promise even though she could no longer feel anything for her friend.
Human Circle Blood Mage: The human was filled with guilt and grief for what had happened and so even though her friend was no longer the same she still spent time with her. When circle fell (due to the events of DA2) the two women managed to fight their way out and flee Kirkwall.
Er sorry that ended up being a bit more long winded than I intended. In any case the reason I think it would be more interesting to have another character along with the tranquil character is because we never really get to see a tranquil mage with a character that actually cared about them before they were made tranquil (well without getting all stabby anyway) someone who's not willing to let go even though the person they knew isn't really there any more. Also it would be a good opportunity to explore the idea of trying to cure tranquility, you know if you're into happy endings.