I love Doom... But I think he shouldn't be the "build up" villain. And I am sorry but I would find, unless they come up with a REALLY unique angle, that Galactus is a retread of the singular villain aspect of Thanos only less interesting.
I think something like the Skrulls in a Secret Invasion arc is different and more a challenge and more interesting than just another singular all powerful villain. There's more complications. And for those that think that the CM movie totally put the kibosh on the Skrulls as antagonists, don't be too sure. Yeah, THOSE Skrulls were sympathetic but even Talos talks about having done terrible things in the fight against the Kree. This isn't an either or sort of conflict and so to with the idea of the Skrulls coming to Earth. We don't know what further transpired with the Skrulls as a whole in the MCU post CM. Or even what happened before that. Talos and his immediate group might be the victims of the Kree but we don't get the view of things from the leadership of the Skrulls as a whole. And again, who knows if the Skrulls are all hard luck cases or if Carol after finding them a home that more militant voices don't take control. Lots of ways you could still introduce Skrulls as a threat to Earth, while keeping them understandable even sympathetic in the MCU. I mean... What if they are looking at the end of their race for whatever reason? Say Galactus isn't the big bad but we see his actions kick off something that eventually affects Earth? Say the planet that CM brings the Skrulls becomes a haven for all persecuted Skrulls and they thrive for years and years... Only to have Galactus come and destroy that world and now they are homeless again. That's gonna change things socially and politically among them.
I can also see Kang as a big threat. If time travel/alternate timeline shenanigans are a part of AEG as seems to be the case then that's a perfect backdrop to justify Kang, who brings up all sorts of complications and interesting challenges to the MCU that isn't repeating the Thanos arc again in some way. A gloss on something like AVENGERS FOREVER with the Immortus side of Kang in play would also be something you could tease out over many films culminating with another big Avengers cross over film.
Again... I don't think Doom should be the build up villain. He should be in the FF film/s as their big bad... BUT... That doesn't mean he should not be a player in any big mega arc. Quite the contrary. Whether the bads are Skrulls, Kang or the Celestials coming to judge humanity, Doom should be a big player, perhaps sometimes allied with the heroes but always with his own agenda, whatever it may be. Much like the original Secret Wars I would have him as part of any story in some way only to emerge eventually as the final boss if you will. That's more interesting than he's just plainly the bad guy from the start that has to be stopped. Have the audience go on a ride same as the heroes, never trusting Doom but knowing he's too much of an asset in a given situation to dismiss aid from.