Anyone who was deep in the throes of the MCU fandom during the release of WandaVision will remember the Mephisto craze. Everyone was theorizing that the character of Mephisto was going to be the next big bad of the MCU, because there were many hints towards him in the show, and he's been a major part of Wanda's story in the comics. Mephisto never appeared in WandaVision, but he has appeared in many fan theories about other Marvel properties since the show aired.
Sometimes the theories are made somewhat jokingly, as a reference to the wild theories about Mephisto during the WandaVision era (for example, a fan sees a red item in a scene and makes a joke that it's a reference to Mephisto), while others seem deliberately planned by Marvel to make people talk and to ensure that the fans are thinking about Mephisto (such as the devil painting that appeared in Loki). However, making Mephisto the big bad of the next Marvel phase simply wouldn't work, least of all because they're seemingly already setting up Kang the Conqueror to be the next over-arching villain of the MCU.
In terms of grandeur, making the devil the ultimate villain is kind of the highest threshold that can be hit. Where would Marvel even go after that? Mephisto as the big bad feels like a move that needs to be saved for further down the line, when things in the Marvel universe have gotten even bigger and crazier than they are now. It might be too much for some viewers to believe that a being such as Mephisto is something that actually threatens these characters when the multiverse and grand-scale magic have only recently been introduced to the MCU.
But who is Mephisto anyways? In the comics, Mephisto is a demon who rules a dimension that is similar to the Christian idea of Hell or the realm of Hades in Greek mythology (and, importantly, Mephisto himself is neither Satan nor Hades, but presents this way to play into the notions surrounding devil figures that people on Earth have). Mephisto's realm is populated by the demons that serve him, as well as the spirits of humans who have died and are now trapped in the bodies of demons. Mephisto seeks to add more of these spirits to his collection by making deals with humans who are still living and getting them to submit to his will.
That kind of character is a lot, and it might be too much to introduce to audiences at this point. It's not as though the MCU is a stranger to mythological figures, but the Avengers having to fight the literal devil might be too much for the suspension of disbelief for casual MCU fans who aren't super familiar with the comics.
This isn't to say that Mephisto can't be used at some point, however. Perhaps in a future phase of the MCU, once the Kang storyline has run its course, they can bring Mephisto into the fold and have him be the next big villain that the heroes need to take down. He could be introduced as a villain or even as a brief cameo in a movie before taking his turn as the big bad, but regardless, it would need to be further into the future for it to work. He could show up in a future Doctor Strange installment, as magic and the supernatural are major elements of Doctor Strange stories and it wouldn't feel like Mephisto was being shoehorned in. He could also appear in a Thor film, if Marvel plans to make any more down the line, as having a devil figure show up in a series that's already partially baked in mythology wouldn't be too much of a stretch.
His story could also be tied in with that of the Scarlet Witch, as in the comics, he's very involved with the lives of her and her children. His soul was actually what created Billy and Tommy in the first place, and he eventually reabsorbed them, taking them out of existence. Obviously, this wasn't how events went down in the MCU, but because the MCU is likely to see Billy and Tommy back at some point, it's possible that Mephisto could be a part of that story. However, this seems more unlikely because this storyline seems like one that will be happening soon, and it's still too early to introduce Mephisto to the MCU.
Waiting a few years or a few Phases to bring in Mephisto would be a good strategy for Marvel to take. Bringing the character in now would only confuse audiences that aren't comics readers, and it would just make the story messy if they tried to have both Kang and Mephisto as major players. It would be much smarter for them to wait a while and bring Mephisto in much later down the line when the MCU is in need of another revival or shakeup. Introducing the devil is certainly something that would take the MCU in a new direction, and it might be the refresh they need in a few years time.