It gotta be Vivienne. Can you imagine the rage that will be unleashed if the guys get less options then the girls? I mean, I wouldn't rage myself, but I can totally see it happening.
Not gonna lie, I'd actually rub my hands together and say "Ehhhxcellent!" if Blackwall's in and Viv's out. I have far more interest in romancing him than Viv, anyway. 
Regarding Solas being straight, I'm not bothered, really. This just means my gay m!Dalish will cuddle with Bull instead, or Blackwall, if he's indeed the last LI, not elf-gated and is into male elves. If he's none of those things, then I guess he'll romance Bull as he's the only other option. EDIT: Oops, I forgot about Dorian. He's also an option, unless he's gated against elves. I doubt he will be, but you never know...