Of those whose LI status and orientation has not been confirmed, the below list outlines my thoughts. I think I may be being optimistic on the numbers though.
If this was to be the case, my first playthrough would be with a female elven PC (mage or rogue) who would romance Solas. I prefer to keep elves together, they are so oppressed by humans that I think it fits best, but if that's not an option it will probably be Cullen.
Varric - available to dwarven PCs only (possibly only female) - Dwarven PCs will also be able to romance other races.
Dorian - avaiable to male PCs only - I'd be very surprised if they chose to make both elven characters gay.
The Iron Bull - available to Qunari PCs for romance and possibly other races for roll in the hay. May also be romanceable for humans and elves but I would be very surprised if he was available to dwarves.
Cullen - Available to female PCs only - there's been no indication that he is bi in previous games, although they could ignore that.
Solas - Male and Female PCs (or maybe not romanceable at all)
Blackwall - I don't think he will be a romance option at all, from his description he is a very committed Grey Warden and I think that would be at odds with him becoming involved with anyone.
For the three remaining, Cassandra, Vivienne and Scribbles, I just don't know, any one of them could be bi, but I don't have any reason to think one would be more than the other. If any of them are to be unromanceable I'd guess at Vivienne, as she seems to have an agenda, and getting involved might be at odds with that.