I'm surprised they didn't find a way to shoehorn Ripley into them somehow.
Well, she's not around yet. We still have one more theoretical movie to cover how the much-larger-than-an-Engineer Space Jockey gets a ship full of facehugger eggs and crashes on LV-426 enough years before the
Nostromo toddles along to find him mummified/fossilized into his chair.
As for the MCU... We already had Skrulls confirmed in Infinity War. I'm apparently a little iffy on the particulars. I thought the entire race was part of the Fantastic Four rights package, not just Super-Skrull. Others say, no it's just Super-Skrull. I thought the word itself was out-of-bounds, hence going with the alternate "Chitauri" in Avengers. I'm hoping Galactus and Silver Surfer get worked into the big cosmic fight -- even in cameo, to be developed later.
Meanwhile, while Marvel has Peter Parker, I would love to see an Ultimate-FF-inspired take on their origin overlapping one of the forthcoming Spidey films, where Reed is another student at Peter's school, who disappears, and rumor is that he's been recruited into a secret government think-tank project called "The Baxter Building", which is odd because that building no longer exists -- it was torn down to build Stark Tower. And take things from there. It'd be a nice late-Phase-4/early-Phase-5 addition, I think.
As for the X-Men... This Marvel Earth (199999) seems to have not had mutants manifest the way they did on other Marvel Earths (616 -- the main comic universe, 1610 -- the Ultimate comics, etc.). We could have them start later, triggered by something recent to the MCU. We could have some escape between dimensions thanks to Infinity War or some future use of the Reality Stone, or Reed Richards' experiment-gone-wrong, or they could even have been here all along, and just were better at staying undetected until Something Happens™... There are a lot of options.
--Jonah