The most important element I'm looking for is Steve as an omega.
Preferred pairings are Tony/Steve or Bruce/Steve. However, I wouldn't mind if Steve's alpha was Clint, Thor, or Sam. Fury and Coulson are probably alphas, but I'm not sure I can really picture them in the situation... on the other hand, I'm not opposed to the possibility if you know one you really like.
No Bucky/Steve please. (I can see why people ship it, but it feels like the wrong side of incest for me)