Doom is incredibly hard to write well if you're going for a "serious" villain, because Doom is inherently comedic. He's a dork in a Medieval looking suit of armor that speaks in the third person, owns a vaguely European nation, has a massive ego, and also built an army of robots that look just like him. On top of all that, he's a guy that named himself "Dr. Doom", so having absolutely no self-awareness must be part of his character.
If you were to make more of an action-comedy film for the F4 and have Doom be the villain in the same fashion Loki was, then you could have a great depiction of him, but any attempt to make Doom "serious" will most likely fail miserably.
The FF aren't a grim and gritty, serious type property at all though. Even at their most serious, there's still a sense of levity balancing it out. You can't treat them like Batman and do a Dark Knight, it's more Indiana Jones in space. Trying to do a serious version of the characters just shows that the writers and everyone involved didn't get why the property works.