Let's not forget that characters like Quicksilver have been both an X-Man and Avenger, so I think characters like him could appear in the MCU. It's probably just a case of licensing agreements.
If Disney pick up the catalog from Fox, it would not be the first time something like that has happened. By this, I mean a branch of the House of Mouse acquiring the rights or taking inspiration from the Fox companies (counting it's days as Fox Film Corp. and Fox Film's corporate predecessor, Box Office Attractions). That is, 1 of the 1st Fox Film releases was 1914's Gertie the Dinosaur in Nov. 1914. This cartoon by early American animation pioneer Windsor McKay was 1 of the 1st cartoons to have a character with a distinctive personality. Thus, it did inspire a young Walt Disney.