There seems to be a matter of rights extensions. If they were allowed to simply include the FF in any X film and include them in the title they are basically extending the rights of two separate franchises with one film. I would think that contracts written at the time are very specific about what constitutes an extension. For instance spinoffs would reset the clock. We saw this with Elektra and DD. There were safeguards in place against such a thing. Certain characters were included with each property. Doubt they could just make Galactus the main villain in Xmen 8.