Another character that would require some extensive make-up alterations to pull-off (and one whose MCU debut is even more hotly anticipated) is Mephisto.
Since we already know Ewan McGregor plays a good villain from Birds of Prey, I think putting him to the ultimate test by casting him as one of the worst antagonists imaginable would make for quite an interesting experiment.
Essentially the Satan of the Marvel Universe (or Marvel Multiverse, more accurately, since he does not originate from one true dimension), this is one extremely mischievous demon who has been known to be responsible for some of the most vile and horrifying crimes against many different Marvel characters, which would certainly be interesting to bare witness to on the big screen in what could very likely be the darkest installment of the Marvel movies in order so far.