If Fox goes forward with that crossover their investors should start questioning if their money is being spent well. Ignoring fanboy feelings on how awesome crossovers are and approaching it from a financial perspective, it makes no business sense. With no merchandise money coming in, you're really just looking at the numbers for the film itself with these Fox Marvel movies. Fox gives a larger cut of the box office to Marvel for Fantastic Four than they do for X-Men. There's no reasonable way that a crossover would see Marvel receiving X-Men money and not the higher percentage Fantastic Four money. Let's say the crossover costs $150 million and makes $600 million. Fox then has to deduct Marvel's FF deal cut. If, however, they do a crossover between X-Men and New Mutants at the same budget it's likely it would bring in similar numbers (no tracking has Fant4stic bringing in the kind of money the X-films have done, so it wouldn't be likely to provide any larger of a boost than New Mutants) and they'd only have to give Marvel the smaller X-Men cut. There's also the issue of actor contracts. Using up an X-Men actor appearance in a movie that will make less money on the investment than an actual X-Men film is not smart business.