I do believe taking Wasp out of the Ant-Man movie would indeed lessen the film in some way. She's been there from nearly the beginning. Plus, not to sound cliche, if she's not the female lead and love interest whose going to fill that roll? Hank needs Jan for balance in many respects.
I think she'll end up in the Ant-Man movie. This is perhaps a misunderstanding of Whedon's intent for her with TA2