Cobie Smulders could appear in S.H.I.E.L.D..
She tells the I Am Rogue website "the commitments to [How I Met Your Mother] don't affect it and I can't really say but there are definite talks about it." ETA: And a Hollywood Reporter video interview with Cobie talking about S.H.I.E.L.D. can be found here (find out whose show it is!).