I'm guessing her return will tie into the new concept of a multiverse created by messing with time. It was recently revealed that James Gunn was a major creative force in planning the next phase of Marvel. I'm glad Infinity War and Endgame didn't just drop a number of major changes with the Guardians in his lap for him to figure out while designing the 3rd film. He's been in the loop on all of it. I wouldn't be surprised if Guardians 3 involved the characters traveling to alternate realities to deal with events that made them such misfits.