Currently in the comic books, unless they retcon again, Scarlet Witch and Quicksilver aren't mutants. I think that was due to them not being mutants in the MCU due to a deal with Fox but of course Disney now owns them. If they want to they could say now that Loki's staff unlocked their mutant powers if they want to go that route.Comic book Wanda has the power to alter reality. I suspect we will see her grieving for Vision take some odd turns. With the tie in to the next Dr. Strange movie, she could use that to bring back Vision and Quicksilver, possibly opening the introduction of other mutants.
Both Vision and Wanda got their powers from the Mind Stone. My guess is they are in a shared reality either created by the Stone or by their combined powers. Although Thanos "destroyed" the Stones, they can't really be eliminated, only dispersed to atoms. So their powers still existed. Also, the stones were plucked from earlier in the timeline for Endgame, so wibbly-wobbly timey-wimey stuff may be involved.I'm guessing that more will be explained during the series. It probably has something to do with her powers. Maybe she watched a lot of old sitcoms.