As far as big screen crossovers go, there are two that will probably never happen. The first is between Marvel and DC, for obvious reasons, and the other is one between the X-Men and the Marvel Cinematic Universe. Despite the fact that the mutants did originate at the House of Ideas, Marvel Studios owns all the characters within the MCU, while Fox holds the cinematic rights to Wolvie and the gang. As such, a crossover just isn't possible, as the two studios don't seem to be very keen on working together.