Before You Read
I'd like you to accept 2 ideas (whether they be correct or not) before you continue reading. 1.) The fact that a crossover film would most likely be a cash grab reaction to the Avengers. 2.) The Fantasic Four reboot won't be utter garabage and will be capable of building up to and off of these kinds of ideas.
Who is Kang and what does he have to do with anything?
A long time ago, there was this guy named Nathanial Richards, see Nathanial hates this other guy, let's call him the most powerful guy to ever come from earth, HyperStorm. Now some of you already know where this article is heading, but for those of you who don't. For now, all you need to know is that HyperStorm is powerful enough to challenge galactus. Nathanial Richards makes HyperStorm his mortal enemy and swears to destroy him, but he can't just do that right away, so Nathanial does some time traveling. Nathanial does some pretty cool stuff throughout his journey through time, including fathering a son named Reed, (that's right, Reed Richards) taking on the role of Immortus and Scarlett Centurion, those two are the least exciting of his escapades though, we will be focusing on his other two. At one point in his life Nathanial finds himself in ancient egypt... as a pharoh, he's called Rama Tut. Rama Tut's most notable achievment is grooming En Sabah Nur, and placing him on the road to eventually become the X-Men's greatest enemy. So Mr. Fantastic's father was not only an ally to Apocalypse, but he helped make him into what the X-Men fear today. Most importantly in Nathanial Richard's life, is becoming the time traveling super villain known as Kang the Conquerer. Enemy of the Fantastic Four. I might also want to note that Kang is a decendent of Dr. Doom.
Days of Fantasic Past
We all know who Cable is right? Nathan Summers, son of Scott Summers and Jean Grey. What you may not know is that Scott Summers and JEan Grey also had a daughter, named Rachel Summers. Whats so cool about Rachel, you ask? Well she is the daughter of 2 incredibly powerful mutants, but more importantly, she has nigh limitless power. How does this somewhat obscure future X-Woman have to do with the fantastic four? Well she is the lover of one Franklin Richards, a telekinetic. These two love birds are quite the couple, and eventually have a beautiful baby boy, named Jonathan Reed Richards...who becomes the intergalactic conquerer known as HyperStorm.
So far, we have a decendent of Dr Doom, father of Reed Richards, friend of Apocalypse, who swears to destroy his great grandson, who is the grandson of 2 members of the Fantastic Four and the X-Men.
Let's say they set up Kang in a Fantastic Four movie, then this could go all sorts of places. Here's my prefered example: Apocalypse and Kang team up to kill Franklin and Rachel before Jonathan can be born, because Apocalypse doesn't want to be over thrown and Kang as we know by now really hates HyperStorm. Well the future X-Men and Fantastic Five might be too much too handle for these time traveling knuckleheads, so they decide to go up a generation, to Reed, Sue, Jean, and Scott. Why not up another generation? Because Kang likes being alive. So the X-Men team up with the Fantastic Four to stop this dastardly duo, but wait. They beat the bad guys, so everything is cool right? Nope, in some great sequel set up, they have inadvertantly caused HyperStorm to be born. Franklin and Rachel cant beat him on their own, so who they gonna call? The Fantastic Four, X-Men, Kang and Apocalypse, who all team up to defeat HyperStorm once and for all, and if that isn't a fitting end to the Foxverse, then I don't know what is.