In a series of intertwined tales, What if? Age of Ultron (2014) explored how repeated attempts to travel back in time might cause the multiverse to come apart at the seams.
As one timeline unraveled, Wasp died inexplicably during a conversation with Henry Pym (then Giant-Man) about his initial plans to create Ultron. After that treacherous machine turned on him and the Avengers, Pym (now Yellowjacket) joined a team of Defenders consisting of Black Widow, Nick Fury, Falcon, Silver Sable, and Shang-Chi (Master of Kung Fu). But Ultron defeated them as well.
Another tear in the fabric of reality brought about the unanticipated death of Thor. The Defenders from before now included technologist Lieberman (a.k.a. Microchip) instead of Pym. While the team faced an army of Frost Giants and the Norse doomsday serpent Jormungand, Black Widow recovered the hammer Mjölnir that had belonged to Thor and earned the title of Thunder God.