Who do you think should be the main villain for MCU's Fantastic Four? I don't think it will be Doom because he's already been the main villain in the previous films.

I totally agree with you! Dr. Doom is way too overused in previous Fantastic Four movies! I therefore propose a preview of Cillian Murphy in the role of Victor von Doom but for me, the perfect enemy would be the Mole Man! he has style, a mysterious side, good origins! He is stylish!
Then we can always look among the unknown and make a little more "cosmic" enemy like Anihilus, Molecule man, Super Skrull, the Terrifics (or even why, not a good version of Galactus preset as a celeste in the Eternals!) .

So we can leave Victor Von Doom for the second or third movie, as this villain has already appeared directly in the previous Fantastic Four movies.