krinsbez on AlternateHistory.com suggested (a long while ago) a scenario where different aliens, representing different alien archetypes, invade Earth. I've started to do some brainstorming on this, and here's what I have so far. My other ones planned are the Brainleeches (brainwashing aliens), the Formics (bug aliens), the Parasite (...parasite aliens), the Elder Ones (guess), the Integrated Systems (assimilating machine invaders), the Others (extradimensional aliens), and the Europans (undersea monstrosities, akin to Cyber's sharks, commanded by an evil alien coral!). I've fused the genocidal aliens with the conventional invaders, the human-aligned aliens with the refugees, and the assimilators with the machines. I may split them, but these are already a lot of factions (not including the Men in Black and other human factions). If anyone has any ideas for names or splitting these alien archetypes, feel free to suggest them!
The Invaders - When the first alien invasion occurred, it was basically what every moviegoer expected. Large, city-sized flying saucers over the world's major cities. No warning from the attack. Lots of alien drones swarming around causing havoc. It was so predictable that casualties were actually not as bad as they would have been, as humans retreated from the cities as soon as the aliens arrived. The Invaders are not the most advanced alien species on Earth after the arrival of several other species trying to take the Earth for themselves for different reasons. However, their ships still pack the greatest punch, and they still have many portions of Earth under their control. The Invaders have no interest in human life, as their plan for Earth is to strip mine it and move on to the next target. This has made them...rather unpopular.
Martian Empire - Otherwise known as Little Green Men (LGMs), these are the second (and more comedic) alien invaders. They originally evolved on Mars, but were forced to flee the planet approximately 65 million years ago after their war with the saurians. They are aligned with the Invaders, playing the Italy to the Invaders' Germany (the superficial resemblance of their armor to those of ancient Rome make the comparison even easier). They like their flying saucers, tripods and death rays, and want to conquer Earth because 1. they want to secure the entire Sol System for prestige purposes, 2. they think it's funny, and 3. for some reason bovine proteins have an effect on them similar to cocaine with a human, so they want to corner the supply of cows in the galaxy. They're also addicted to human snacks, because every good stoner needs munchies. They have spent most of their time ruling their portions of Earth making humans do stupid things, like carving Mt. Rushmore clones for themselves (they've been getting high off cow the entire time). As the name implies, they are small, green, and humanoid, although they lack visible mouths (they speak via voice modulators). However, there are many strains of genetically engineered LGMs, which aren't very little. Their leadership caste are actually very tall and thin, but are rarely seen on Earth because the gravity would break them. In fact, their standard footsoldiers are about ten feet tall and look like they're members of an East German shot put team, but they're dumb as rocks and have to be commanded by the four foot something sergeant with the death ray. They have built an Illudium Q-36 Explosive Space Modulator on the Moon, and are aiming it at Earth. The Invaders don't want them to fire it just yet (the Space Modulator would change the chemical properties of the Earth's minerals and make them useless), but to do so if the Invaders start to seriously lose ground.
Grays - Ayyy, it turns out the tinfoil hat crew were right: there were aliens hiding in Area 51! The grays are a marginalized species that have been driven from their home system by the LGMs and kicked out of every other place they've tried to settle in, finally deciding to crash on Earth in 1948. However, their influence is far more limited than conspiracy nuts believed: most of their technology was too advanced and translation of technical terms only achieved in the early 2000s. Reverse engineering brought doom to Earth: it was the activation of the first human-built hyperdrive that attracted the LGMs to the planet. Gray technology is similar to LGM tech (the grays admit that there was a lot of cross-pollination a long time ago), but while LGM tech is far more compact and efficient, the grays have the upper hand in genetic engineering. With the blessing of the Men in Black, the humans they protect, they have been engineering human-alien hybrids to fight back the various alien invaders and to have weird sex with. About that last bit: gray culture on Earth has been radically transformed by the Sixties counterculture, which they never grew out of. This is why they've adopted a lot of New Age bullshit that they try to use to "enlighten" humanity with (forgetting that these beliefs were a human invention), to the eternal chagrin of the Men in Black.
Slendermen - An early attempt by the LGMs to infiltrate the Men in Black, the slendermen are a human/LGM hybrid that are intended to look like bog-standard government agents. While some of these infiltrators are very good, one of them succeeding in assassinating the President of the American Emergency Government by dressing as a prostitute, most of them reflect a lack of LGM understanding of human physiology and psychology. As their name implies, slendermen are very tall and thin, always wear a pastiche of a suit (in fact made out of folds of skin; they have to wear other clothing over it), and sometimes lack facial features. Those that do look passably human always move strangely, or stand still for unnervingly long periods of time, and when they speak it sounds like a dog trying to imitate a Russian trying to speak English while chewing gravel. Since they're largely failures, the LGMs have dumped them all over Earth, where they have formed their own communities that sometimes collaborate with humans.
Systems Protectorate - If Earth was Eastern Europe and the Invaders are the Nazis, these guys are the Soviet Union. The Systems Protectorate is a collection of several sapient species that dominate the Orion Arm, and have been at war with the LGMs for centuries. Originally presenting a friendly face to the nations of Earth and assisting in their resistance against the LGMs, when Protectorate forces actually arrived on Earth they proved to be tyrannical conquerors. It doesn't help that they've been xenoforming the territories they have "liberated from imperialism." Their technology is advanced, similar to those of their Invader rivals. Despite their obvious malevolence, many human groups have become active collaborators with the Protectorate, although even these see the Protectorate as the "best possible alternative" as opposed to actually supporting them.
Tsoukaloids - Named after the infamous Ancient Aliens host as a joke (it stuck), these enigmatic aliens are responsible for the development of a global human civilization but were forced to retreat behind the Moon after a cataclysmic war with the Elder Ones. Their technology is advanced, but pales in comparison with what the other aliens are using; what was impressive for Neolithic mankind isn't so impressive to the other spacefaring species. Their technology does resemble that of early man: lots of floating pyramids, laser cannons that are crafted out of gold and shaped like cobras, and their craft are armored in super-dense material that superficially resembles sandstone. A lot of effort has gone into making Tsoukaloid technology look in this way, which reveals the Tsoukaloids' greatest weakness - they are obsessed with theatrics. Pyramids? Large and impressive. Gold and jewelry everywhere? Everyone loves bling. Human sacrifice? Audiences love blood and guts. Their entire culture has degraded, revolving around looking impressive, and when faced with the biggest threat to their existence since the Elder Ones, they don't hold up. After the invasion of the Moon, they have reestablished a foothold in Egypt and are aiding the human resistance.
Saurians - The original sapients from Earth, these bird-like (not reptilian humanoids, thank you) "aliens" were driven off Earth 65 million years ago. They've spent the rest of that time hiding from the Invaders, Martians and other local bad guys, finally returning to Earth after they heard of the Invader...invasion. Saurian tech is actually fairly advanced, and capable of going toe-to-toe with the Invaders, even if like the Tsoukaloids it looks like something out of The Flintstones. They are masters of genetic engineering, and have engineered giant war monsters based on the genetic stock they took with them millions of years ago. Yep, they have kaiju. The saurians are aiding the various human factions, particularly those affiliated with the MIB, but expect to have portions of Africa and South America reserved for themselves in the event of victory. They have already secured these areas, so good luck trying to convince them otherwise.