Emily VanCamp's Agent 13 has featured in two Captain America films now, but despite being an integral part of the character's supporting cast, Marvel have offered no indication that they'll be using her going forward.
And indeed, VanCamp herself has said that Sharon Carter won't, in fact, be featuring in the upcoming Infinity War, which was meant to feature just about every Marvel character ever, supposedly. That's really frustrating, particularly so if this does turn out to be Steve Rogers' last stand, as it stands to reason that we won't see the character in any other MCU franchises going forward.
That's a shameful waste of her potential, and of VanCamp's talents too. Sharon Carter is such an important part of the Captain America mythos and, while Rogers himself might bite the bullet come Avengers 4, there's no reason why Carter can't feature in some future Cap-themed series, be it Bucky-led or otherwise.
Better still, let Carter run things on her own. She's one of S.H.I.E.L.D's finest agents and a fine alternative to Black Widow too, if there's no route in sight for that project to get off the ground.