Now that Marvel have the cinematic rights to the X-Men and the Fantastic Four, they've got a pretty nice problem on their hands. According to Bob Iger, the plan is to bring the formerly Fox-owned franchises into the existing MCU - including, quite strangely, Deadpool - which, of course, inspires all sorts of questions about integration.
How will they explain the lack of mutants to date, for instance? Will we get origin stories for Marvel's first family? Could alternate universes be used to introduce the characters as residents of other distant worlds?
It's all very intriguing and it will no doubt be the dominant conversation once the Infinity Saga is out of the way. But then, that's assuming that the Infinity Stones and Thanos couldn't play a role in the route into the MCU for the new characters. Despite the fact that the rights deal was only worked on relatively recently (at least that's the narrative we're all TOLD anyway), there's every chance that some groundwork could be laid in Avengers 4 to set up their appearance (even without them actually appearing).
3. The Significance Of The Reality Stone
So how could that happen? It all comes down to the Reality Stone, whose powers might be the narrative skeleton key that Kevin Feige needs to bring in his other properties. After all, you don't get more useful a narrative tool than an actual McGuffin that allows you to make whatever sweeping changed you want to the very fabric of your franchise's reality, which is precisely what the Stone is capable of.
Though we haven't seen it manifest properly in the MCU, the comic book Reality Gem is capable of extraordinary acts. It allows the user to alter reality to the limits of their imagination, as well as the laws of physics, while also commanding the ability to resurrect the dead, confuse enemies by temporarily distorting reality or conjuring entirely alternate realities. When combined with the other stones, it allows the user to alter reality on a universal scale.
Now, it is already being assumed that Thanos will use the Stones to successfully kill half of existence, but there's still the question of what ELSE the Gauntlet might be capable of and what happens after Thanos is defeated. If the films follow the comics, he won't be the last person to wield the collected Stones (Nebula took it over when he left his physical form arrogantly) and there's every possibility that the Gauntlet could be used to accidentally change reality entirely at that point.
That would certainly suit Marvel's agenda to make everything fundamentally different in Phase 4...