Well, here's the thing, and my take on it:
If Rudd plays Lang, and we're still getting the "veteran/mentor" Pym and "padawan" Lang story that Wright/Cornish have been promising from the beginning, then I could see some method to the madness here. Namely: if Lang joins the Avengers as Ant-Man, then Pym is still free to join as....
Giant Man.
And I like that direction a lot. It would look more like the classic 616 Avengers. That's one thing people keep forgetting about this whole Ant-Man shindig: Ant-Man represents only the smallest portion of Hank Pym's career. He is FAR better known as Giant-Man and, to a lesser extent, Yellowjacket.