Not that there was ever much doubt, but Cobie Smulders says she'll be returning to play S.H.I.E.L.D. Agent Maria Hill in The Avengers: Age of Ultron.  Speaking to the Calgary Hearld, Smulders says:

"I think I’m in Avengers 2, but it is very top secret.  I hope I’m in Avengers 2. I didn’t get my invitation yet. Well, actually, they know I’m in it but they don’t know to what degree I’m in it…I am really excited to read the script because they are introducing Quicksilver and the Scarlet Witch into the Avengers universe and the bad guy [Ultron] is going to be very interesting."

Hill didn't have much to do in The Avengers, and while we don't know the size of her role in Captain America: The Winter Soldier, I doubt she'll have much to do in Age of Ultron.  If Agents of S.H.I.E.L.D. gets picked up for a second season, she could be a strong addition to the cast (assuming she'd want to do it).