Some people tend to believe it will be Galactus but I hope not, he's not a "villain" per say. But seing him in the MCU would be awesome along with Silver Surfer !
I'd love to know more about the Beyonders but I'm pretty sure it's still too soon for them to appear in a movie yet.
Dormammu has already been seen but dunno about his potential as a villain in the MCU, I mean Thanos was the very best choice for that because he have a certain humanity that spectators can relate to, but Dormammu ? Eh.. No keep it in Strange movies and we're ok.
Venom movie is not part of the MCU (correct me if im wrong) and it would be nice to incorporate him, great villain ! (yeah I know he's more of an anti-hero)
I'd love to see Mephisto, I know very little of him and I'm very fan of mystical/supernatural/cosmic being like him and that would bring Ghost Rider to the MCU in addition !
Not a villain (nor a hero) but do you people think OAA will one day appear on screen in the MCU ? I think not but eh he appeared to Spider Man once so it's possible right ?
Let me know your advice and all ! Im also here to learn as I do not really know the Marvel Universe so well and Im eager to hear your suggestion !
PS : sorry for my english I try my best