Honestly, Fiona pays for her crimes by having to live her life.
That woman is a mess... correction, was a mess.
I think it would be great if people who sided with the mages have to end up fighting Fiona in the next game because she sided with Solas.
Please no, we don't need her going up another rung on the Mary Sue ladder by putting her right beside an ancient elvhen god. I would prefer she not be part of the revolution/major conflict at all. God knows that she's already been involved in too many "special" positions on Thedas. Grey Wardens who are connected to the blight, her being the only "cured" grey warden, then she became Grand Enchanter....we don't need "Fen'Harel's General/Lieutenant/Valued Agent" getting added to the list.
No...If we are going to have her fall or grow, I think it would be best if it culminated in the form of a quest chain or something that emphasizes how her past decisions may come back to haunt her. Like being assaulted by a sect of mages who want her forcibly removed from her position, or maybe the taint comes back to her in some form or maybe the wardens get involved with her. Basically anything that has to do with her position(s)/past and really emphasize how even she has flaws. It might add a bit more depth to her.