Good. The former films completely screwed up Doom twice already. I'm hoping for Annihilus, personally, but there are a great number of excellent villains they could pick from the FF comics.
The Leader (already introduced way back in The Hulk), Super Skrull (I know used in Secret Invasion, but easily could come through one of the other extremists), Frightful Four, Galactus...
However, the most obvious that would follow where the MCU has been going is Kang. With the multiple Kangs and multiverse can already wide open, he seems the most obvious way to move them from the 60s into the "present" MCU.