Well, now it appears as though fans have an early look at what that cast might look like, as well as some of the stories implied by their roles.
IMDb lists Maximiliano Hernández as S.H.I.E.L.D. Agent Jasper Sitwell--something that's not entirely surprising considering the actor has reportedly hinted himself that he might be involved with the film. Coupled with the presence of Nick Fury (Samuel L. Jackson) on the page, it's possible the whole page could be written off as speculative, with S.H.I.E.L.D. agents simply being slotted in on IMDb because of their past feature film work. Cobie Smulders and Clark Gregg aren't here, though--and then it starts to get oddly elaborate.
The site lists Adam Campbell as playing Dr. Walter Lawson, a character that the fan press has already speculated would be a great fit for S.H.I.E.L.D. Lawson has connections to the Kree and Captain Marvel (Mar-Vell briefly impersonated the character), and died in his first appearance, suggesting that perhaps the pilot could see one or more of these rumored characters go while setting up the premise of the show separate from the premise of The Avengers and the rest of the Marvel Cinematic Universe.
Campbell certainly looks like he could be inserted into the role of Captain Marvel if they wanted to play out the impersonation bit for a while--and he ought to be comfortable with the Comic-Con audience at this point, being that he's married to Glee and Heroes standout Jayma Mays.
Another familiar Marvel name that fans might see there is Tandy Bowen. The alter ego of Dagger (of "Cloak and Dagger" fame) will reportedly be played by Samantha Lemole, a veteran of the Saw film series. Cloak and Dagger is a premise that has intrigued Hollywood on and off for a while, with press and fans speculating that they could be made into their own TV series with a little tweaking. Introducing Bowen into S.H.I.E.L.D. series may be a road to that, but certainly is a way to give fans a little something to look forward to--if they know what they're looking for.
There's also a listing that Peter Outerbridge, of Nikita and Land of the Dead fame, will play "Governor Drookes," a character for whom there is no comic book analog as far as I know.
The IMDb page for S.H.I.E.L.D. may not be a perfect source of information--it lists the show as a TV movie, for example--but it's pretty rare that something of this magnitude would make its way onto the site and not be immediately pulled. That said, try not to take all of this too seriously; if you'll remember, there was a point when Cobie Smulders was listed as a Skrull for a while before the release of The Avengers.