I always liked the first two FF movies too. Doom was pretty bad in them... but the core origin/background/relationship of the family was rock solid. I definitely put them in from time to time to rewatch... but the last one? Never again :P
I know what I want them to do with Doom... I don't believe they will, but I know what they need to do. They need to go Full circle. Darth Vader was supposedly designed after Dr. Doom.... well now they need to take tips from Vader.
NEVER EVER Show his face. Keep it in the helmet. No 'pre-mask' Victor hanging around... No Mask getting ripped off for Shock Value... Keep it hidden. Make him the scariest most intimidating character in any room he's in. Find someone with an epicly deep voice to do the lines. They don't have to be the same actor as walking around in the suit. He's powerful. He's confident. Dr. Doom should be all of this. Don't even bother with a big name fancy actor who needs to flash his face in front of the camera.
Frankly I would do the same with Kang.. but i'm less emotional about Kang. I think Avengers Forever was the only real Kang story that I ever read. I had the Secret Wars toy when I was really young, but knew nothing about him. He's really a non-entity to me. Do whatever they want with him...