Until BioWare explicitly states otherwise, I will continue to hope/believe that Cullen will be a DAI companion.
As to the other options that have been discussed, I have all the books but haven't read them yet, so meh to any book character, (considering a large portion of the player-base may not know who these characters are either). Especially one who is mentally a child, as stated by D.G.
I really find it hard to believe that there are people who want a romance with such a character.
I've discovered through these forums that there seem to be those who would want to romance a rock or something equally ridiculous, so.....whatever floats your boat, I guess.
If they do include book characters as companions, I hope they take the time to really explain their backstory in the game, and not just assume players know who they are. If they make such assumptions (I'm looking at you, Kai Leng), it could really make those characters seem shallow and one-dimensional to players who are not in the know.