Honestly I think you could fit the FF into the MCU a lot easier than you could the X-Men. Either make it a period piece like the first Captain America movie or do an Ant-Man where they were already out of the hero game. Of course when talk of the FF come up it's usually just to get Doom or Galactus. For the X-Men you'd have to do a reboot or some DOFP type timeline screwing to weave them in. Aside from that how would the MCU really benefit from having the X-Men? Does anybody really want to see AVX in live action that much?