It's probably best that they just try and develop the cast they have instead of trying to bring in superheroes.
That's probably the best way, though I bet we do see superheroes pop up here and there during sweeps or some such event. Heck I could even see a superhero show up because he has a film coming out soon (Captain America 3 coming out next week? Why not have him guest start on SHIELD to set up the plot of the film).
Actually now that I think of it, I would love that idea. It wouldn't be anything that you had to see in order to enjoy the film, but it'd be a nice bit of supplemental material all the same.