I've been kicking around one.
Basically it's a tabletop version of Invader Zim, Destroy All Humans, and X-COM where you specifically play as alien invaders.
The alien threat should most likely be a confederation of different alien races so the players have a lot of options for what to play, but some species would be naturally prone to animosity or better suited to all the same race. This would depend very heavily on session 0 play style to familiarize players with their options and the mechanics. Obviously something more like a xenomorph would be a lot harder to pull off and should have more specialized dice rolls for their character to succumb to instinct. Others could be grays, whose objectives would focus heavily on abductions and study. You really could mine quite a lot of pop scifi for player types and objectives. I would draw the line at lovecraftian entities simply because they're too powerful. I would however save that for NPC alien threats. Because I would also have modules structured around rival alien factions with different invasion methods and even more hostile attitudes, possibly even ones with active human collaborators.
The emphasis should be on stealth, and the gameplay very objective-based. Humanity is very much a Humanity Fuck Yeah caliber of enemy mixed with some comic stupidity. The players should be an outlying force relative to their alien affiliation, possibly they crashed by accident, maybe they're the seed invaders meant to gather intelligence, but they shouldn't be the armada or mothership. Effectively blending in or hiding out would be critical to achieving their objectives. Players might get lucky on not being discovered but rest assured once it happens they will not hesitate to escalate the conflict from rednecks all the way up to MIBs if players get careless. This would also be some of the fun, I think some PARANOIA-flavor could be added in where the players are to some degree being set up to fail or get caught, and have to do damage control while attempting to bail on their mission. Actually, it would make a lot of sense for the setup to be that the players crashed their vessel and have to work towards being evacuated from earth undiscovered.
Honestly the more I think about it the more scenarios and angles I see possible. There could be peaceable aliens the players could be, and the campaign would focus on establishing contact with the CORRECT type of humans, and dealing with the politics of that, or even having to fight off an invasion from another collection of aliens.