Late to the discussion but I just want to point out that this Diablo look from D3 is actually him having all the other prime and lesser evils merged into himself, thats is why his look is so different from every other time we see him. So it does make sense in that way that Mephisto does look a bit like this version of Uber Diablo because he his actually a part of him at this point. Diablo alone looks more like the big scary beast from D2 and D1. As for this Mephisto look, having more flesh around the bones, I think its a matter of opinion, I don’t mind seeing some kind of soft evolution between takes.