Originally Posted by
The Overlord
You what could be interesting? The Fantastic Four go to Hell.
I am being serious, apparently, Mesphisto is the reason why Diablo is so long-lived (and he would not grant a wish like that, a longer life span that would deny him Diablo's soul for now, if he did not have a reason for it), Mesphisto is the one being Doom would hate more Reed and Mespisto having several run-ins with Silver Surfer. Diablo doing Mesphisto's bidding and making the FF end up in Mephisto's realm instead of somewhere else could be interesting, Mephisto is one of the most powerful villains around, but he is different from Galactus in how he operates, he is far more manipulative and cruel, Galactus does what he does to survive, but Mephisto is pure evil. You have the Surfer and Doom involved and I think the FF had a few run-ins with Mephisto, but nothing major, but there are tons of plot threads with FF related characters.
In Unthinkable, Doom could not hack it as a mage, because as a wizard (good or evil) you serve the magical beings that give that power to you, they command you, rather then you commanding them (Doom could not accept that and got dragged to Hell) which is why I think Baron Mordo and Dr. Strange are better wizards then Doom, they know this deal and accept it, so that limits how skilled a mage Doom can be. But Diablo, he could totally be Mephisto's servant and thus be more skilled as a wizard then Doom. It is a darker exploration arc than others, but it does put the FF against a different type of villain and you can easily involve other characters like Diablo, Doom, and the Surfer, it can a major epic arc.
Sure the FF have had run-ins with Mephisto, like that cross over mini from 1987, the fact that he is a big factor in so many FF related characters, demands a real rematch, exploration takes the ultimate wrong turn.