Sean Bean as Dr. Henry "Hank" Pym
The original Ant-Man, a biochemist from the 1960's who discovers a set of subatomic particles through which the size-altering formula is developed. Sean Bean not only resembles the character more than Aaron Eckhart or anyone else does, but his English Yorkshire accent could deliver witty lines akin to other Edgar Wright movies (Shaun of the Dead, Hot Fuzz, Scott Pilgrim, The World's End) He was also the Bond villain in GoldenEye (1995) so he can turn the audience against him when things start to get "domestic" between Ant-Man & the Winsome Wasp (I know some people don't want to see this, but it is an important part of who Hank Pym really is, and it'd be an insult not to show us his full spectrum of flaws)
Rupert Grint as Scott Lang
That's right, Ronald Bilius Weasley could very well be the young protagonist in this story with a character-arc spanning a thieving electronics expert to a full-fledged Micronaut when he "inherits" the legacy of Henry Pym. Though Grints filmography is dominated by drama he's had comedy roles in Thunderpants (hilarious) and Cherrybomb across from Robert Sheehan so there's something about this casting just feels RIGHT. He shares a lot of similarities with Simon Pegg's physical mannerisms so Edgar shouldn't have a problem directing his acting, perhaps Simon could coach Rupert on doing comedy so that Grint's dramatic elements don't take over.
Mary Winstead as Janet van Dyne "The Winsome Wasp"
This one's a no-brainer, she was great as Ramona Flowers and could be even better as this other pre-conceived comic book character who represents the strong 1960's retro female, takes a liking to an older man and soon regrets ever shrinking down to his level.
Robert Sheehan as Eric O'Grady
The role of The Irredeemable Ant-Man requires an Irish actor who is also ridiculously funny if its to be done correctly, and with enough re-imagining of the source material Lang and O'Grady could be best friends a la Pegg/Frost and who knows maybe even become the first English-Irish comedy duo. Grint and Sheehan have a natural chemistry both on and off screen because they aren't too similar or too different, Robert is obviously "the funny guy" of the two with his role as Nathan on BBC's Misfits as a prime example of this.
Richard Gere as Vernon van Dyne
The 1960's storyline will be intentionally over-glamourized and pulpy compared to the chrome, nihilistic, and British future storyline, so in this case Richard Gere is the right guy to take Sean Beans place of dying within the first 30 minutes of the film.
Nick Frost as Elihas Starr
This character is less of an A.I.M. villain, more of a bumbling meddler with comic relief sensibilities, and Nick Frost can play this part better than Simon Pegg, but if MODOK is involved in the film they'll know who to call.
in fact...
Simon Pegg as George Tarleton
Now if MODOK is to make any appearance in a grounded universe, he needs an actual purpose that differs from his origin in the comics. I have a few ideas for how a large hovering head/torso man could be used to practical effect; he is a security machine designed to track down and kill every shrinking individual. Dr. George Tarleton represents the oppressive nature of Advanced Idea Mechanics and their quest for ill-gotten knowledge of Pym's particles, so maybe the always-likable Simon Pegg isn't a bad addition to this comedy-action movie.
Thank you, that is all.