I would make for a good game, but then if we wanted historical accuracy (except for the Hitler assassination part), we'd ALL have to have knowledge on the subject.
Extra ideas: the aliens live can for several hundreds of years in a lifetime, that's why the story takes place over 300 years (space travel isn't fast)!
Starting at WW2, and then ending at 22xx, that's where you go into hiding!
WW2 (and all that knowledge you find out about) isn't the only part of the story, you also have to protect your 'people' from invasions from other aliens (different species), and stop other wars between other species.
And: global warming was caused by several Alien spacecraft hovering over Earth, but having a spacecraft invisible creates a lot of heat, the CIA realize this and order you to move the ships away, but the rest of the human population don't know this and keep 'combating' Global Warming, and thus create a 'Nuclear' Winter.
The whole Earth freezes over and so the Aliens have to get every living thing from Earth to Mars, as well as most of the water (this is where the Humans find out about the Aliens).
Mt. Olympus Mons is no longer the galaxy's highest mountain.
BTW: this whole idea just came to me after im not a people person73 inspired me with his post.
We should have a vote on whether we want this idea or something else.