The X-Men aren't just coming to the MCU, they're already here.
In the last few years, the X-Men have begun making their way into the Marvel Cinematic Universe in the most unexpected of ways. Evan Peters made a surprise cameo in WandaVision, Patrick Stewart stunned audiences with a return as Charles Xavier in Doctor Strange in the Multiverse of Madness, Hugh Jackman is set to reprise his role as Wolverine in Deadpool 3, and most recently Kelsey Grammer shocked fans with a return as Beast in The Marvels.
What was so special about Beast's appearance in The Marvels was that it seemed to mark a major turning point for the X-Men within the MCU. His cameo clearly took place at the X-Mansion, he was joined by fellow X-Men hero Binary (a variant of Monica's mother, Maria) and he name-drops Charles confirming this is a new reality we've not yet seen within the MCU -- one where the X-Men are quite an established team.
While it's unclear whether the universe we saw in The Marvels is the same one connected to Fox's X-Men franchise, it's clear that Marvel is building toward something major and it seems incredibly likely more classic X-Men character will soon be making their way into the mix. There's no telling who might follow Beast into the MCU next, but there are a few characters, in particular, we want to see get the MCU treatment after all these years.
Note: This list is strictly looking at characters from Fox's original X-Men franchise films, not the rebooted franchise that followed it years later.