Scarlet Witch has an insane power spike at a climactic moment of a future movie, and dropping to her knees with tear stained cheeks and super-hero chaos happening all around, she whispers...
I think it's a little too late to go "There were mutants the whole time in MCU, we just never talked about them!"
Reality bends and breaks and folds onto itself and burns to white, and as we fade back in, Jackman and Fassbender and the rest are there. Wolverine bites down on his cigar and asks Tony "who the hell are you?"
And the credits roll.