Just toying with some designs. These could have worked well as Neo concepts too
They are -
The Xillians - A race of humanoids who had long ago eschewed emotion for pure logic. However, instead of turning down a path of moderation and civility, they resorted to pure coldness. They see humanity as a threat, a race of half-monkeys with technology that far outstrips their ability to use it. Better to conquer them and put them in their place before they destroy the solar system.
The Kilaaks - Only a handful of these silicon-based life forms are left after their former planet was destroyed millions of years ago. Living in the warm cores of the floating asteroids, they have become obsessed with doing the work of their lord and eventual "savior," the mighty King of Terror...
The Nebulans - The little invasion that could! They are a shadow people, inheriting a planet that was barely usable, even for an insect. Now they're playing a game of chance with Earth, balancing their plan to wait out humanity's inevitable extinction with the desire to get a jump on things with their super-weapon, Gigan!
The Simians - These beings that arrive on Earth do not represent the majority of their race. They are arms dealers, military capitalists and black market entrepreneurs. Their home planet is slowly circling a black hole, and their plan is to conquer Earth, enslave the populace, and sell this broken (but functional) world to their leaders back home. Then they can retire to some moon to drink cognac and puff some fine cigars to their hearts' content.