I can imagine that Cassandra is a pretty safe bet. I believe (correct me if I'm wrong somebody...) David Gaider is writing two characters and two romances, with Cassandra being one of those characters. I can't imagine the writers would write romances for characters they didn't write themselves, but then I'm not in the industry so I can't really say.
I'm pretty sure he's the writer for Solas, the elf mage, as well, though in that thread he seemed to turn down the notion that Solas is a romance. Although he usually writes the romances, maybe we shouldn't assume that it's a romance just because Gaider wrote it.
I'm guessing there will only be four romances total. Two male, two female, all available to all players. If they add another one or two, I'll be surprised. Most likely they'll be Cassandra and the blonde elf woman and... for the men, I do not know. Cullen isn't confirmed, and if Cole is in-game he will NOT be a romance (thank goodness--Cole has the mind of a child, so while a spirit romance might be interesting, I'd rather it be a character whose mind is far more mature). Iron Bull is a possibility. The Warden doesn't strike me as one who would be available to romance. Varric? Perhaps.
I'll be shocked if there are three males and three females available for romance, but it's a possibility. Then there'd be Cassandra-Vivienne-Sera, and... whoever the three male romances will be, probably including Solas but who knows since Gaider seemed to indicate otherwise? And the males are the ones I'm actually interested in.