I doubt the Captains will be romantic interests. I mean, Carol is a brash Boston girl from the '90's. How much would they really have in common outside their (very different) military backgrounds? At most, I see a potentially fun Air Force vs. Army/Yankees vs. Red Sox rivalry developing there. That said, I do think Brie and Chris would probably have chemistry since they've been friends for a while.
But if she must have another Avenger as a love interest, I vote Sam. They're both Air Force, both on the snarky side, Sam would definitely hit on her (it took him all of 2 seconds to hit on Widow, lol), and I could easily see them walking that line between butting heads and flirting on a regular basis.