There are some interesting characters from previous games that could possibly appear in a new game. I know that seeing Jowan, Alain, Feynriel and Fenris hanging around Tevinter would be interesting, but they can possibly be killed in their respective games, and although BioWare has gotten away with that before, I think that's lame. So I'd like to focus on characters that are alive, no matter your choices. There are some characters that I can think of:
Lily - Former Chantry sister and Jowan's lover. Last seen being jailed at Aeonar, but later her cell was found empty. She's a sister that can fight, much like Leliana, but unlike Leliana we don't know where she got her fighting skills. Also, after the whole thing with Jowan, what does she think of mages now, specially Blood Mages?
Eleni Zinovia - The talking statue at the basement of Ferelden's Circle Tower. What was she doing there? And since she can still speak, chances are that she's still alive and someone from the time of Andraste still hanging around is very interesting if I do say so myself. Maybe she could even be turned back into human (and I'd like her to be a woman with a shaved head, just like the original statue from DAO).
Lord Otranto - In DAI, if you romance Josephine, at some point her parents arrange a wedding for her with Lord Otranto, and you have to duel him for her love. The thing about that duel scene from DAI is that he is a match for the Inquisitor, a master duelist indeed! And even if you don't romance Josephine, she seems annoyed by the arranged marriage anyway, so I don't she would marry him no matter what.
Charade Amell - Gamlen's daughter. This one isn't very important nor interesting, it's just her connection with Hawke and the fact that's she's a Friend of Red Jenny.
Tallis - I think we've all had enough of her and maybe getting Felicia Day to voice her again could make production complicated, but hey, she's still alive.
Sandal - We still don't know anything about that dwarf. Maybe he's the one and only dwarven mage or even a born tranquil? Also, he seems powerful (he did kill all those darkspawn by himself, including an ogre) and Boddahn won't live forever (specially considering that he actively seeks trouble). An orphaned Sandal would add some drama to him, who knows how he woud react to Boddahn's death.
Anyway, anyone knows any other interesting characters that could return on a future game?