I think Vivienne will be the last LI; I personally don't see Blackwall as a romance option since he is a grey warden and has that sense of duty.
I don't like to judge until I actually play the game but I'm kinda let down with the romances for straight males. I usually play RPGs as a straight male human warrior and the LIs offered in DA:I for straight males doesn't appeal to me that much:
- Cassandra: This could be the only one I'm interested the most though I don't like how her face has changed since DA 2, but we'll see in the actual game.
- Josephine: She doesn't attract me at all, plus we know little about her.
- Vivienne (if she ends up being the last LI): same as Josephine.
Dragon Age: Origins appealed me more with only 2 romance options for males. I romanced Morrigan of course. She is better than all those 3 LIs together in my opinion.
The only female that actually attracts me in DA:I is Sera, and she is only available for other females... so either I make a female character and romance Sera or I make my male character and I don't romance anyone at all.
Obviously, my perspective could change when I actually play the game and I get to know the characters, but that's my first impression though.