Marvel should probably cancel this film now. It's Scott Lang instead of Hank Pym. Pym was horribly aged and even Pym is a B-list hero who's only interesting when he's on the Avengers and not as a solo hero.
Marvel mentioned having scripts for Captain Marvel and Blade and Feige has mentioned being a fan of Black Panther. Why not use one of them since they're far more popular heroes and none seem likely to appear in Phase 3?
I remember Feige once saying that Ant-Man was only being made since it was Edgar Wright. Now that he's gone, what's the draw for a potential audience for a character who audiences may have trouble taking seriously?
As far as I can recall, none of the Ant-Men have had great runs on the comics like the above three. Christopher Priest's Black Panther, Marv Wolfman's Tomb of Dracula and all three books starring Carol Danvers (Claremont, Reed and DeConnick) are well-regarded by fans. I can't think of a single great, essential solo Hank Pym work since Stan Lee's Tales to Astonish. Nearly all of his stories since 1965 have just been Avengers titles. Hell, he even lost his own book to Hulk at that time. Now when Ultron is no longer part of his character arc, he's not really needed for any role.
Amongst the other choices, BP has been a member of the Avengers for decades and was part of the team during the Kurt Busiek run along with Jonathan Hickman's current Avengers work, Captain Marvel was part of the team during Kurt Busiek's run and even became the field leader under Bendis and now Blade was the most recent Ronin and just joined the Avengers a short while ago. Any of these three can fit. Alternately, I wouldn't mind a solo film for War Machine or Black Widow.
Phase 3 right now is
Ant-Man
Cap 3
Doctor Strange
Thor 3
Hulk 2
Avengers 3
Why not bump off Ant-Man and give fans a character that we really want?