Mephisto needs to make his grand debut in a live action 'Triumph and Torment' adaptation alongside the MCU Dr Doom. With either Strange or Wanda serving as a co lead.
I'm not opposed to him showing up in NWH or MoM though.
Best theory I've heard is that Bruce Campbell is playing Mephisto (and he was Mephisto in disguise throughout his cameo appearances in the Raimi movies) and that's him pretending to be Dr Strange looking for a Spider-man who would be willing to make a deal with the devil. Or he was influencing Strange and Peter and interfered at the spell at the last minute. Or the spell didn't go the way Mephisto/Strange intended to either.
I wouldn't put anything past Marvel after the guy Obadiah yelled at in IM1 came back as part of Mysterio's crew. lol