It’s always possible Monica Rambeau wound up in the Fox X-Men universe. That franchise always played fast and loose with continuity, so it wouldn’t be out of the question for Beast to look different. There’s still the case of where Binary came from since she wasn’t in any of the Fox “X-Men” movies, but that could also be hand-waved away. Monica may have arrived in a universe that’s similar to the Fox-verse but wholly its own, allowing the MCU to play around with continuity and change things it sees fit. Either way, “The Marvels” mid-credits scene could factor directly into the upcoming “Deadpool 3.”
“Deadpool 3,” which sees Deadpool (Ryan Reynolds) team up with Wolverine (Hugh Jackman), looks like it’ll involve ample multiversal concepts while functioning as a sendoff of sorts to the Fox Marvel movies, including the “X-Men” films and “Elektra.” The duo could travel to other dimensions or simply provide one last look at the film series that helped launch superhero movies into the cinematic stratosphere. That means they could wind up in the dimension Monica was in.
It would be nice if the Merc with a Mouth could give her a ride back to the main MCU continuity, but it would also totally be within Deadpool’s character to leave her there or even agree to take her home but forget. However, it may be a bit anticlimactic if Monica got back to the MCU so quickly after leaving, so she may be there for a while. At the very least, maybe “Deadpool 3” can provide the universe number she ended up in.