ABC picked up the Joss Whedon series which is a continuation of the extremely profitable Marvel Cinematic Universe. Clark Gregg's popular character Phil Coulson will be headlining the series along with several new characters. I will make a list of Marvel characters that should make their Marvel Cinematic Universe debut on the upcoming series later, but this is a list for existing MCU characters that should make an appearance on the show. As cool as it would be to see the Avengers on TV I imagine they would not be happy to learn that Fury let them think that Coulson died (also I do not see Robert Downey Jr. shooting a guest spot). These are characters that could easily transition to the small thing for a guest or recurring role. I am counting out Samuel L. Jackson's Nick Fury because I think that it is a given that he will appear (or at least do a really awesome voiceover).
10. Galaga Guy from The Avengers
I doubt that a nameless S.H.I.E.L.D. agent who plays eighties video games during a world crisis would have what it takes to make it to Level 7, but it would be fun if they made a reference to this gag.
9. The Cellist from The Avengers
This was an unseen character in The Avengers that Coulson had previously had some sort of relationship with. As the star of the series it would be nice to show more of Coulson's personal life. Although Coulson's existence is a secret it could make a fun episode if he has to save this cellist.
8. Doc Samson from Incredible Hulk
One of the most popular actors on ABC is Modern Family's Ty Burrell. Burrell had a bit part in Incredible Hulk as Leonard Samson. I am not sure what Marvel's policy on The Incredible Hulk is, but even though Banner was recast and reinterpreted I assume that it is still being treated as in continuity. S.H.I.E.L.D. could go to Samson if they needed a psychiatrist or general smart guy to consult with.
7. Happy Hogan from Iron Man movies
Jon Favreau's Happy Hogan is a real fun character. It was a lot of fun seeing him with more screen time in Iron Man 3. If S.H.I.E.L.D. needs to contact Stark Industries it would be easy to get their forehead of security a guest spot. Agents of S.H.I.E.L.D needs to tie to Iron Man somehow.
6. Winter Soldier from Captain America: Winter Soldier
The movie is not out yet and I do not know how it will handle the Winter Soldier, but the work Winter Soldier does is right up S.H.I.E.L.D.'s alley. Depending on the interpretation of him he could be an enemy or ally to Coulson's crew. He could even join the series' main cast after the movie is released.
5. The Leader from Incredible Hulk
One huge advantage that this series has is that it can pick up dangling plot points from the movies. One of the intended sequel hooks for The Incredible Hulk was that Samuel Sterns turned into the gamma powered Leader. It does not look like any movies will follow this but it is a perfect premise for an episode of a TV series. The Leader is not a special effects heavy villain or a physical threat, he could play a big bad that complements Coulson's crew well. Tim Blake Nelson would make a great recurring villain that would work on TV.
4. Jasper Sitwell from Thor, The Avengers and Marvel One-Shots
Sitwell is one of the more prolific S.H.I.E.L.D. agents in the movies and comics. I am sure he will be used, but it would be fun to see his legendary patsy.
3. Erik Selvig from Thor and The Avengers
Stellan Skarsgard is a well respected, prolific actor but he is not too big of a star to turn down TV work. Much like Coulson this man has seen and done a lot. It is doubtful that he could return to a normal life after The Avengers. I imagine that the character would continue to work with S.H.I.E.L.D at least as a consultant. He would make a fun guest or even recurring character that would help with ratings.
2. Stan Lee from Everything
Okay it is a given that Stan cameos in everything Marvel, but I still wanted to mention him. And I am cheating in his inclusion because he is not one character he is many. Agents of S.H.I.E.L.D. should continue this tradition. He could play one recurring role (a great opportunity for Stan to play a big character) or a great gag for the series would have him cameo as a different character each season. S.H.I.E.L.D. was his concept and audiences love spotting his cameos, Marvel is almost required to keep this tradition.
1. Maria Hill from The Avengers
Cobie Smulders has said she was unable to join the series because of her commitment to How I Met Your Mother. But when the series finally ends next year (man they dragged that story out way too long) she would be free and would make a great addition to the cast. Really hope to see more of her Maria Hill.