A few plots I'd love to get - (these say steve but b-e can be other characters too.)
A. Pre-serum, Bucky and Steve were together, Bucky promising after the war they would officially start the next chapter of their lives. Things never go according to plan but after Steve rescued Bucky, they renewed their commitment towards one another and started to make new plans. Then Bucky fell from the train and once again that all shattered into pieces. Skip forward to WS and the two encountering one another yet again - third times the charm, right? Only Bucky didn't have any memories of anything but still felt very, very drawn to Steve regardless.
B. Pre-serum, Steve was the Omega and Bucky was the Alpha but the serum and the programming for the WS change things all around making Steve a new Alpha and Bucky an Omega. Hydra figures Bucky will reject Steve and that will be one way to break Captain America but things do not go according to plan at all.
C. Dom!Steve and sub!Bucky. Pre-serum or post-serum.
D. Soulmates AU with Bucky not remembering the bond/about soulmates at first. So, WS based.
E. Omega!Bucky but with his true nature so tied up and restrained, he doesn't remember it until it gets triggered.