Blackwall:
- He looks like an old experienced Wynne kind of character, but age just for age is not an argument, specially because we can create old looking characters as well;
- His description states that he took the life of a Grey Warden as a choice above any other option in his life, so he values that a lot. But so does Vivienne with all her ambition and either she or him are an LI. If one couldn't be a love interest because of strong determination, the other could not in the same way.
- We've seen grey wardens as LIs before. Alistair would prefer to be a grey warden more than anything else, specially considering the effect of Duncan on him.
- Grey wardens were central on Origins and on Dragon Age 2, act 3, they appear telling about some more important things happening. Dragons everywhere on Inquisition and archdemons? Well, maybe that means that Blackwall have a big role on Inquisition. The fact that they didn't say much about grey wardens may as well imply a big plot on the subject, thus making Blackwall a great LI for the player character.
Solas:
- He has a lot of appeal on his background story with big ties to the main storyline so far. That alone make him a great love interest for the player character.
- Between Sera and Solas, dalish would prefer Solas for sure. Actually, he is the best and almost only option for dalish protagonists than any other character.
- A lonely reclusive guy that suddenly appears to help the world...A lonely guy...Yeah, game on!
Vivienne:
- She is someone hungry for power, putting herself before anyone else. Maybe there is a personal story there behind this iron behavior, rich enough to be explored as a LI. Something like "I don't trust anyone, but you may change me" sort of thing.
- That she is the only female character left to be a LI is a big argument, strong enough to make me believe that she is a LI.
Between Solas and Blackwall, well, if I had to bet, I would go with Solas, but knowing that I'm risking a big backstab from the writers with Blackwall.