I really like the idea of using AIM as the primary antagonists in the initial Ant-Man film. Especially since all indications point towards "To Steal An Ant-Man," you could keep all the core elements of the story and add Advanced Idea Mechanics to the fray. With AIM holding the one person capable of saving the life of Lang's daughter, he steals the Ant-Man suit from Pym and proceeds to wreak havoc on the organization's plans.
At the outset of an Ant-Man film series, I don't think there's an absolute need for one antagonist in particular, but adding MODOK in a sequel would be an excellent idea. It's a shame that the Cosmic Cube was taken back to Asgard at the end of The Avengers, since that would be a great way to explain some of the mutations that could occur at the hands of AIM. The next best explanation is that AIM gets their hands on some of the cube-derived technology developed by SHIELD & nearly reverse-engineers the source (but the side effects are far less predictable, which could lead to the creation of MODOK).
One other thing that might be interesting, but some people will probably hate it - out of all the discussion coming from the reveal of A:AOU, you have leeway with Pym's character because his most iconic storylines have been transposed elsewhere. What if he had a hand in the creation of AIM in the Marvel movie universe? With a doctorate in biochemistry, Hank Pym would have fit right in with some of the things that early AIM projects in IM3 were trying to accomplish. But after he saw what Killian was really about, he abandoned the organization once the morals began to shift. This way, he can still harbor some of the feelings that make the character of Pym who he is: the guilt of being a founding member of an organization that morphed into a greedy, power-hungry collective that dangerously pursues human evolution & also the belief that he was helping humanity but in the end created a monster that comes back to haunt him.
While it's not the same as unleashing Ultron on the world, it's a way to write in some of the character traits that make Hank Pym the tragic figure he often portrays. Plus, it ties into the existing canon of the established films, which can strengthen Ant-Man's place in the Marvel movie universe.
