Mine would take place at the end of Multiverse of Madness.
During the events of the movie, Wanda does something major to break or merge different realities. After Wanda and Strange “fix” whatever ends up happening, something gets missed.
Cue after credits scene. An unseen woman can be heard going through labor. Its present day. Doctors and nurses are talking in German. You hear gasps and sounds of shock. One nurse screams. Cut to newborn room with babies in bassinets. We see several curious onlookers gawking through a window at one baby in particular. Camera focuses on a normal baby, and slowly pans to the right. Next baby is normal. The last baby, as it slowly pans over him, is blue. His hands only have 3 fingers. His feet only have 2 elongated toes. A devil like tail is seen. The baby opens his eyes, stares at the camera, and disappears in a cloud of black smoke