My guesses, (in order of likeliness in my opinion, and first two for each gender are the firm guesses):
Sera, (token elf and 'easy' romance, though probably less obvious about it than Zevran and Isabela)
Cullen, (just because the Cullen fans would revolt and come charging into the Palace Of Edmonton screaming for Gaider's head on a stick. The way things are going, I would probably join them, their influence is insidious)
Cassandra, (I think they know people want a woman warrior LI. Plus she's a royal, and would fill the royal LI slot)
Dorian, (though I have doubts he will actually be a companion, my guesses are lacking a bit of magic, especially when mages are pivotal at this point in time)
Vivienne, (Cassandra narrowly beats her actually in my guess. She was nearly First Enchanter, implying age, her silly hats may indicate a mild case of female pattern baldness, but she be so classy! And I find it weird we haven't had a mage LI who hasn't threatened us with fireballs to the face yet)
Mr Grey Warden, (Interesting concept art, but he's old looking, so same problem with Vivienne really. Also the problem with the Calling, though potentially adorables/heartbreaking in romance! More I think about it, more I want him and Vivienne together, hehe!)
Iron Bull, (A qunari romance would have it's challenges after all. Particularly with a female dwarf, though I imagine he might enjoy that actually.)
Varric, (Bianca's the only girl for him and with the fact she may well still be alive....)