Let me start off by saying that this is totally possible. Not just by production standards, but purely on a storytelling perspective. After The Avengers' battle with Thanos, anything could happen after the universe is set back to normal-ish.
Some people called me loony for suggesting a soft-reboot after Infinity War, but why not have all Marvel characters under the house that cheddar-loving mouse built? I'm not talking about everything changing, but a few minor tweaks, here or there! And their are two characters (besides the mutants) that honestly would make Marvel way better!
The biggest question: How many movies would be released in a given year? Seriously? I'm already starting to feel somewhat fatigued by all these movies, and if the money stops pouring in -- then is it worth it, cause baby I'm worth it, sorry! Or will this help keep the amount of movies down?
There are some grumblings that we might see the X-Men join Marvel's MCU in a crossover event -- hooray! If this happens, then guess what? This just made the Inhumans super-interesting! Yeah, The Inhumans are extremely powerful, but they're loners. I really don't want to see a movie detailing their history being all that interesting. But if the Mutants come into play, then gee-golly, we got some stories!
SHOULD X-MEN JOIN
Before X-Men: Apocalpyse, the answer would have been a resounding "NO'! Personally, I just wasn't impress with the movie and many others felt the same way. Honestly, we don't really trust Fox, we trust Marvel. That goes a long way at the box office!
A partnership with Marvel Studios would without a doubt be nothing but a good thing for the X-Men franchise -- except for one of the most popular characters in the world: Deadpool. Are you willing to trade his ability to do anything he fu*king wants, just to see him chilling Spider-Man (no, yes, no, dammit). But wait, would they really have to give that up?
I'm not sure. It'd be weird if he appears in a Spider-Man movie and some kid likes him -- then they go see his solo movie and see him being pegged by his girlfriend. If this partnership goes down, it would seem that they would have tone down Deadpool, only if he were to make regular appearances in Marvel's MCU.
So, everything else really doesn't have to change. There are multiple ways to include them into Marvel's Universe and it'll benefit both studios. If Fox's already planning on releasing a sh*t-ton of movies, why not have them be under the banner of Marvel Studios.
There's not really a downside to either sides and it's a bigger plus for the fans. I'm not quite sure how a crossover would work, but they should just forego that, and have them be directly part of The Avengers!
WHAT ABOUT THE INHUMANS?
What the f*ck are Inhumans? That's what the majority of casual fans will say the first time they hear that name. Also, they will automatically think they're X-Men ripoffs and it'll probably make them even less popular (Is that possible?). But we know differently. They're a fascinating aspect in the Marvel Universe and Agents of S.H.I.E.L.D barely scratched the surface of their mythology.
Basically, they're Kree experiments and they're super-powered individuals who lived among themselves for very a long time. They have their own culture and sadly, for the most part have less cooler names than the X-Men. Also, they could become valuable allies against the fight against Thanos, if we find about them before Infinity War. They have a Royal Family and their King is named Blackbolt. He can decimate a planet using just his voice. So, yeah, they're not someone to f*ck with.
WHY WOULD IT BE BADASS?
Honestly, everyone deserves a chance to go home. Marvel doesn't need X-Men or Fantastic Four, but this will allow Marvel to function at full capacity! Plus, as an avid comic reader, I'm sure this will even make the comics better!
Y'see, Terrigen Mist is what transform humans into Inhumans. But, after a bit of retconning this actually ends up being deadly for mutants. So, for the most part the X-Men leave Earth and find shelter somewhere else. And after huge amounts of Terrigen is released on Earth, there's a population increase for their kind. They have plenty of interesting characters, but I'm not sure how the audience will react towards them.
This would be so awesome to see this play out on the big screen. Because of as now, I'm not quite show you incorporate The Inhumans into all the events that takes place on Earth. I'm not talking about an all out war between the two (I'm sure things will escalate), but this would be a great problem that they must overcome. It will make both of their groups vital in future events. Plus, The Avengers will have to be there to keep peace!
WHY IT WOULD BE REALLY BADASS!
Lastly, Marvel will get back Silver Surfer and Galactus! Did you think I forgot about them? That right there should make everyone happy! That means Fantastic Four as well, but we'd rather see the Silver Surfer chilling with Star-Lord, or sipping cocktails with Tony! And plus, Marvel would keep Galactus in his regular form; instead of what Fox did. Yeah. I'm sold.
I hope that it's not just a crossover event and the X-Men are here to stay! There are so many opportunities, and Fox has already announced quite a few movies over the next few years. So, I'm sure I'm with the rest of you and would like to see them under the banner of Marvel Studios. It'd still be awesome to see them crossover from another universe, or do what Spider-Man did! And for The Inhumans, well, we'll wait and see!