A Who/ENT crossover would've been great! Logistically and financially, I don't think it could happen. I don't think they'd have the resources to make it happen, or the financial ability to make it worthwhile for Paramount. Too many obstacles.
Maybe if they made a large budget Dr Who movie? But, if they were to do that, not sure if I'd want it to be a crossover movie. Novels would probably be the best place.