A female love interest that doesn't revolve around her sexuality or false masculinity in order to be a strong character. I'd like not to see more of an Isabela type that plays up the sex angle to ridiculous degrees (unless we get to have a male that does the overly sexual thing too- at least spread it around, heh). Also, while I really do love Aveline as a character, and would have been interested in her as an LI as well, she came across as too masculine to the point I could have seen a male in her place and nothing would be different.
Women can be strong without playing up their sexuality to distract and without being overly masculine.
Meredith is a FANTASTIC example of a strong woman that doesn't appear like she was first written for a man to play the role. Had Meredith been a companion, I can seriously see some of my characters truly being interested in her. She is strong, but not mannish, and certainly would never have used sex to get her way.
I have trouble liking Isabela because her sexuality is so in-your-face, and the innuendo (or, actual fact, going by a cutscene with Anders) that she's also got STDs just really.. way to play the stereotype of the loose woman with no morals. Why not laugh it up with a male LI needing to go get his infection cured, instead? Though Seneschal Bran did go see Anders for the same thing, I'll give you that, but he wasn't an LI, so it's not exactly the same.
A woman like Meredith is exactly the kind of female LI I'd like to see.
In short, I found Aveline too stereotypically masculine, Isabela was way to hypersexualised, and Merrill way too childlike to really want to romance any of them. One of my characters did take Aveline's flirt options in one playthrough, simply because he liked pushing her buttons a bit and playing around, since he had a sarcastic personality, but none of my others did (and not because I knew it wouldn't work- I knew that from the start when I read a few blurbs about the characters as I met them in game).
Really, just make a template of Meredith, that'd be a hella fun romance to explore.