When I was postulating what we want to see from Marvel Studios in its return to Hall H, one of the things I said was surprise casting announcements beyond what we already know, and Eleven joining the MCU would certainly fit that bill. Obviously, if true, Marvel would have preferred it not be spoiled early, but it should still be exciting for the Comic-Con crowd, for whom I imagine there is a lot of MCU/Stranger Things fandom crossover.