It feels a bit odd, though, because in Captain America: The First Avenger, the subtitle was a description of Cap himself. So this makes it sound (to the uninitiated) like Cap is the Winter Soldier. Might work better if '40s-style title conventions were still in use; then they could call it something like Captain America and the Winter Soldier or Captain America Meets the Winter Soldier.
OR they could watch the movie and be surprised when the Winter Soldier isn't Cap and is in fact revealed to be someone they thought long dead?
But y'know, Captain America Meets The Winter Soldier And Is Surprised To Find That's It's Bucky (His Mate From The First Film Who Fell From A Train) And Then They Have A Fight Because He Has Been Brain-Washed But It All Turns Out Okay In The End
could be the way to go if we're pandering to the 'uninitiated'.