I'm not sure he's really NEEDED... There was a time that he was biggest baddest threat in the Marvel universe. Now we've got a 2-part movie dealing with Thanos and the Infinity Gauntlet... Who SCHOOLED Galactus in the original mini-series.
Galactus really would be a step backwards as a threat. If they wanted 'giant dude who may destroy the earth'... they could always go Celestials? Same principle. Personally, I agree Doom would be better addition. I love him in the Avenger cartoons. Also, ever since the original Secret Wars comics and toys... I've always considered Doom an 'Everyone' villain as much as a FF only villain.