Thanos Who? At This Point in MCU, It's Mephisto Who Seems Inevitable
So far, there has only been one Marvel project that has indirectly confirmed Mephisto's existence, Agents of S.H.I.E.L.D. There's a problem, though: that show is not considered canon in the MCU.
Mephisto's name has been tossed around in MCU fandom for quite some time. After first getting excited about his possible appearance in WandaVision (only to be played), fans are still flirting with the idea of the devious Marvel villain making an appearance in future projects.
Many believe that Mephisto's introduction is inevitable. If Kang the Conqueror is defeated at some point, the MCU will need a new Big Bad, and who says it cannot be Mephisto? In that sense, Marvel could always go back to Agents of S.H.I.E.L.D. and declare it canon as soon as they want to introduce Mephisto (especially since AoS' Ghost Rider implies that the hellish villain is bound to become canon).
Also, many fans note that the MCU is actively exploring a more mystic side of its content, moving away from space and science. Demons, mutants, and witches may all indicate that the supernatural side of Marvel may be coming to the big screen sooner than we expect.
So far, though, the MCU has been very cautious with it. Even though WandaVision was littered with Mephisto hints, all we got in the end was a Ralph Bonher joke. At the same time, Agatha Harkness is expected to return in her own solo show, probably exploring Marvel's witches and magic, and thus possibly Mephisto.
Recently, rumors have been circulating online that Sascha Baron Cohen is being considered for the role of Mephisto, although this has never been confirmed by Marvel Studios or the actor himself. Before (and if) Mephisto is introduced, we have Kang the Conqueror and the multiversal mayhem he will inevitably cause in the MCU.
We will first see Kang in action when Ant-Man and the Wasp: Quantumania hits theaters on February 17, kicking off Phase Five of the MCU.