Marsden’s on the docket, Namor is in play. Plug in a few of the ancillary X-Men returning, and maybe we see the Five take on Doom. A version of it happened in Hickman’s Secret Wars specific to Cyclops.

It could be a way to give Marsden his Garfield moment, pulling a failure from his movies’ continuity (Cyclops dying to the Phoenix) and giving him a redemptive, growth moment.
What are the chances we get some iteration of the Phoenix Five in the MCU?