The YouTube channel Film Theory has posited that Mephisto will NOT be the big bad of Phase 4, or even this series, based on one thing...
CHINA
Supernatural, ghosts, demons,etc, are a big no-no there. Being the 2nd largest film market by a lot, China has held a lot of sway in how MARVEL has chosen to present it's films in that country, going so far as to add 4 minutes to IRON MAN 3 featuring popular Chinese actors, & even showing Tony Stark specifically drinking milk during a time when there were fears of China's milk being tainted with mercury.
At the 2019 SDCC, Scott Derrikson, director of DOCTOR STRANGE & DOCTOR STRANGE 2, said on stage that DS2 would be closer to a straight up horror film than anything MARVEL had done before. Rumors began then that the villain would either be Mephisto or Nightmare. Then just a few months later, Derrikson left the project due to 'creative differences' & reportedly not having enough time to properly prep the film, but after the film being pushed back twice, he still didn't come back to the project & Sam Raimi was brought in to direct.
Going back to 2019, relations with the Chinese censors & worries about the box office were cited as the reasons MARVEL backed off of the original ideas. That being taken into consideration, I can see where Mephisto may be a road longer than MARVEL is willing to walk right now.