I don't know if I want to go for cute and light-hearted, I just don't much care for romances when one person can't accept who the other is at the end of the day. So maybe qunari warrior, but side with mages to keep a little tension there. Isn't there a warrior spec she has issues with?
It's... a two-way street. Sera doesn't completely accept a very elfy Lavellan, but a very elfy Lavellan doesn't accept Sera for who she is either. And the worst of it actually comes from Lavellan depreciating Sera, rather than the other way around - you can be incredibly verbally abusive to her as a Dalish, if you want. Still, these are all dialogue choices you consciously make if you want the romance at its worst; if you choose not to push for these arguments, Sera and Lavellan's relationship can actually be pretty positive for both of them.
Anyway, of the warrior specs, I think Sera tells you to not get all uppity on her if you take up Champion, and gets a bit creeped out about the whole drinking dragon blood if you take up Reaver. Don't remember what or if she says about taking up Templar now, but those are all just one-liners that never come up again. She teases you about taking up Assassin as a rogue, too.