I agree, that sounds AWESOME. As for the FF coming to the MCU, well Fox has the rights until 2021 if I'm not mistaken. I don't see Fox just outright selling the rights to Marvel but I see two scenarios.
1. Fox doesn't do another FF movie, since the last three movies have been horrible bombs both critically and at the box office. The FF brand from Fox is tainted and I don't think Fox investors want to see them finance another FF movie that will lose them $100 million+. The rights will just revert back to Marvel by 2021, just in time for Phase 5 to start in pre-production.
2. Fox does another FF movie, not wanting to make money off of it, but to keep it out of Marvel's hands. Look, we all know an FF MCU movie would be a success. I have faith in Feige and his team. I bet Fox does that, too. They may be stubborn enough to make another FF movie, even if it doesn't perform well, just to keep the rights. Remember, Fox and Marvel, actually Fox and Disney, despise one another. I'm sure Fox isn't too please still that they lost Star Wars to Disney. So I'm sure they are a lot of sour apples at Fox who could care less about the Mouse House. It wouldn't surprise me one bit that Fox will try to keep the rights just to spite Disney.
Of those two scenarios, obviously the second one is more dramatic but I have to believe that the executives at 20th Century Fox aren't interested in doing another FF movie. Even though the FF rights also have a lot of cosmic characters associate with them. I think in 5 years, we'll be hearing about an FF movie from Marvel Studios in production once the rights revert. Fox will have X-Men forever and I think that's fine; for now at least. I enjoy the X-Men movies but I don't like the FF movies but I know I would watch one if Marvel was in charge, not Fox.