I also think that looking at all the info we have Solas is just in front in likelihood of being a LI, but my logic may be wrong.
I fear that making Varric a LI could cheapen his character somewhat as the Bianca story is unresolved (sorry varricmancers its just my opinion
) . As far as I'm aware the events of DAI are not over a particularly long period of time like DA2, so if the Bianca story is to be resolved within the timeline, even if they don't end up together makes a romance with Varric problematic.
Varric getting together with the inquisitor would surely be a bit ... soon, regardless of the outcome?
If Bianca's dead, he'll be grieving,
If Bianca appears and tells him to sling it, any romance with the inquisitor will kind of be a rebound.
Other potential problems I have with the idea are...
If Varric just decides he loves the inquisitor more and forgets Bianca it would seem to be ooc
If Bianca's story is not addressed at all it would be totally lame.
I genuinely can't think of any good story reason for him to be romanceable other than to make fans happy, which is a fair reason, but I would prefer to keep him as a bro'
We know so little about Blackwall that it is hard to say, but reading his blurb and interview, he doesn't seem to be a great candidate due to his commitment to the Grey Wardens and his duty. He seems to be very focused and it seems likely that he is likely very selfless, and may not be looking for any romance, especially with one outwith the Wardens. Two wardens together makes some sense, but a warden getting involved with someone who is not a warden creates problems (difficulty fathering children, having to go the calling at some point, all the secrets) *shoosh Zevran!*
Reasons Solas wouldn't be a love interest are fewer IMO. His commitment to his studies is the only one that leaps to mind from reading about his character. The other possible reasons are not story related - PW already has one LI (but that is one out of three...), BW might be wanting to shake things up by not having an elven LI (but why would they do that, elven LIs are popular
)
Sorry for the ramble... I should go.