It just seems to me you're assuming DeLancie would have turned it down or demanded too much, and that's why a movie with Q couldn't be done and wasn't done.
If Khan worked and the Borg worked, Q could've worked, I think. Q was prominently featured throughout TNG's run, from start to finish. He was high profile and, I think, known. And it wouldn't take long to "explain" him to anyone in the audience who didn't know him.
As for Q being too "magical" or "fantastical," heck, movies of all sort are chock-full of such characters, and not just sci-fi.