Jason Wyngarde (aka Mastermind) of the Hellfire Club wants to use his powers to seduce and corrupt a beautiful mutant, so obviously he chooses his former teammate, Scarlet Witch. Even in the early days, he was intrigued by the potential of her powers, and he assumes that she has gained greater control after being an Avenger for so many years. So Wanda starts having these weird visions of the past around the same time that Mantis is putting the moves on Vision. Then the armored Hellfire Club dudes show up at the Avengers Mansion in an attempt to kidnap Wanda. In effect, the Hellfire Club takes Kang’s place in Avengers #129. The Avengers at the mansion at that point are Thor, Iron Man, Scarlet Witch, Vision, Mantis, and Swordsman, but let's say that Thor gets called away to deal with a threat to Asgard. What happens next?
Kang is contemplating the mystery of the Celestial Madonna, and decides to check his math one last time. In the process, he notices the emergence of an extremely powerful female mutant shortly after the anticipated discovery of the Celestial Madonna and determines that his previous calculations were off. So instead of taking his Macrobots back to the 20th century to kidnap Mantis, Scarlet Witch, and Agatha Harkness, he instead travels to a certain mansion in Westchester. In effect, the story set-up for X-Men #129 is replaced with the story set-up for Avengers #129. The mutants at the mansion at that point are Cyclops, Storm, Wolverine, Colossus, Nightcrawler, Phoenix, and Professor X. What happens next?
If possible, both plot lines will persist in continuing along the lines of the originals. So the Hellfire Club will make a second attempt to get Wanda (like their attempts to get Kitty Pryde and Dazzler), and Wanda’s full reality-shaping powers may get unlocked along with the nasty side of her personality. The Imperial Guard may still make an intervention, etc. Likewise, Kang will make at least two more attempts to get Jean Grey, with the Legion of the Unliving and his own temporal duplicates.