According to James Gunn, there's no reason to fear what Kevin Feige said about Phase 3 being a conclusion to the MCU meaning we won't see a Phase 4 (because some fans were apparently scared). The director - who is set to be a big presence in the creative meetings behind Phase 4 (or whatever it's called) - took to Twitter to confirm that what he meant was that the end of Phase 3 would close out the current Infinity arc. From there, we'd get a new major story entirely.
But what could that be? There are lots of possibilities from the major comic book events to choose from, but it remains to be seen which Marvel would be most drawn to.
Some things are impossible. As much as we'd all like to see an Avengers Versus X-Men crossover, it's just not going to happen. Likewise, Marvel Zombies won't happen as an over-arching narrative (though we can all hope that they start doing feature-length One Shots for that).
But that doesn't necessarily mean that we won't see some cross-over with rights holders outside of Marvel Studios. We should see Namor, for instance - as part of a bigger narrative, probably - and there may be some opportunity for Fox and Marvel to share SOME characters. It's just not likely they'll be Professor X and his buddies.