Other characters who need to be in this show:
Rick Jones: No appearance in Hulk movies for the Hulk's bestest pal? For shame.
Amadeus Cho: the seventh smartest man in the world!
Jennifer Walters: pre-She-Hulk. Then, mid-way through the season, make her She Hulk. Ruffalo shall guest.
Matt Murdock: Hey, Marvel's about to get Daredevil's rights back. Why not defy expectations and have him appear here first before he gets a movie or TV show.
Damage Control: If they can't get their own show, they can still be a presence and also show that the Marvel universe is excepting it's super-heroes as more than a string of isolated incidents.
Jimmy Woo: Then we can have the Agents of Atlas appear later on.
Willie Lumpkin: Because Stan Lee needs to cameo.
And most importantly:
The Great Lake Avengers: Because, duh!
As for big season-long villains:
Baron Strucker: I don't think he's big enough to be a Cap leading villain. But he'd make for a great SHIELD villain.
Scorpio: An actual Nick Fury villain! And Fury's brother. Maybe they should reference Extras and cast Laurence Fishburne.
SWORD: Yes, techinically SWORD are good guys, but SHIELD might not know that at first and it's a pretty common Marvel cliche to have two good guys meet and fight each other before fighting the real villain. Also, Joss Whedon created SWORD.
Terminus: Technically the poor man's Galactus, but it would be a pretty big deal for the series.
Zodiac: Cause let's face it, they are never going to appear in the movies.
Writers who need to do episodes on this show:
Peter David (has TV experience and the man mixes comedy and drama well)
Ben Edlund (do I need to explain why?)
Jane Espenson (Again, do I?)
Jonathon Hickman (Great sci-fi writer and did a SHIELD series a few years back... about the organizations true origins)
Grant Morrison (Joss is a fan of Grant and Grant does have experience writing a well recieved play. I'd like to see his mind-bending stuff on TV. A long shot, but still)