When Evans wants to retire or take a break (I hope he goes for a break, I adore him as Steve!), I would love to see Bucky Cap.
My reasons:
01) I would rather see the character of Bucky take over the Cap mantle than a recast of Steve so quickly after Evans leaves (if he does). It's going to hurt when Evans leaves, because like RDJ, he's BECOME Steve Rogers. If they can get Evans to come back after a decent break, even more reason to have Bucky as Cap. No need for a recast then.
02) Sebastian Stan, IMO, is one of the MCU's most talented actors, whom I believe is more than capable of being the lead in a film. Bucky would be a different Captain America, but that wouldn't make it less interesting, or inferior. There's a lot of depth to be explored with the character of Bucky Barnes, especially post Winter Soldier, and him taking up the mantle from his best friend who he feels is so much better of a man than he is (given his past) that he struggles to honor and do proud would make for a fantastic character driven movie. And Stan would kill it.
Regardless of whether they go in that direction or go in a completely different one, I think Bucky's one of the characters with a ton of potential and Stan is amazing, and Marvel would be foolish not to utilize him as much as they can.