Dude called Praetorian over on RPG.net started a thread asking for ideas on what kind of alien invasion to throw at his players in their superhero campaign, and I posted a couple of replies that were so excessive I figured I might as well reuse them here.
Longpost is loooooooong.
Reasons for the invasion
Colonization - Pretty straight forward: The aliens want some lebensraum, and they don't mind taking it from somebody else.
Conquest - This is distinct from colonization in that the aliens definitely want to keep the humans around. Hell, maybe they don't even plan on moving to Earth themselves. They just want to rule us, using humanity for slave labor, or establishing an economic hegemony over us, or maybe they just want to be worshipped.
Slavery - Similar to conquest, except they only want the people, not the planet.
Resources - They've run out of (or just want more of) something they need and we've got, like water or air or some such. They don't really need to conquer the Earth, necessarily, just take a devastating quantity of the good stuff and go.
Food - Okay, this technically falls under "resources", but I'd say that if the aliens came to Earth to eat us--especially if they're only interested in humans, not all the other tasty creatures we've got here--it results in a pretty different scenario. Especially if, instead of just grabbing a few thousand people and running, they want to actually take over and run the Earth like a cattle ranch.
The McGuffin - They want something from us, but it's not a question of basic need. Maybe it's some powerful or sacred artifact that they have to ransack the earth to find, or maybe they're seeking the source of our superheroes' powers.
Just plain theft - The aliens want our stuff. They want to steal our luxury goods for sale on the intergalactic black market, or they want two of every creature on Earth for their zoo, or their emperor is fascinated by our artwork. Hell, maybe they're obsessive collectors with interest in some weird, specific thing, like cars or guns. Maybe they want all our information, so they're stealing books and servers.
War for war's sake - Maybe it's only the fight itself that they're after. Maybe the aliens have some cultural inclination (or religious duty) to go out and beat up on other civilizations. Hell, maybe they even had to sit back and watch humanity become more advanced for a few thousand years before we became worth fighting. In fact, maybe they've quietly helped our development in preparation for the big event, both by sneaking us new technology and by provoking wars.
Sport - The aforementioned Predator option. I'd say this is distinct from war for war's sake in that it's a more individual kind of thrillkilling, rather than a civilization vs. civilization thing. Maybe it's some kind of religious or cultural rite, or maybe it's literally a sport, possibly even televised back on the homeworld. Maybe there are even competing teams, each trying to rack up more kills than the other, or maybe one team is even tasked with defending the humans. Hell, maybe it's not even about killing people, but they just fuck up our shit in the course of their volcano-surfing contest or whatever.
Extreme xenophobia - Due to some very bizarre and horrible religious or philosophical dogma, the aliens cannot allow other intelligent life to exist. Anywhere. They started firing up the war rockets the moment they caught our early radio transmissions.
Fear - Like xenophobia, but a little bit less irrational: They want to exterminate (or cripple) humanity because they're afraid we are or will eventually become a threat to their own race. This might be because we're warlike and almost advanced enough to come and invade their world, or maybe our capacity to produce superhumans makes us potentially far more powerful than them, or maybe we're just some kind of cultural threat: They don't like the ideas their own citizens are getting from viewing our television signals.
Altruism - The aliens are here to take over for our own good! It's just the green man's burden to conquer and civilize less advanced species! They'll rule us and exploit us, sure, but they'll end all war and sickness, too! And isn't their culture just better than ours? We're only resisting because we don't know what's good for us!
Shapeshifting - Maybe they use it to disguise themselves, Skrull-style, or maybe they just use it to kick ass, regenerate, and adapt to whatever the heroes throw at them.
Telepathy - A pretty standard trope just to let aliens communicate with people. Of course, at higher levels of power, you start to get mind reading, mind control, subtle mental suggestions, illusions, etc. Hell, if they've got enough range, maybe the aliens don't even need to invade physically!
Mind control - This deserves its own section, as it's not necessarily dependent upon telepathy. Maybe the aliens' whole invasion plan involves the construction of some vastly powerful mind control device, or putting drugs in our water and subliminal messages on TV, or kidnapping important people and implanting stuff in their heads. They might end up doing all their real work through human puppets. The heroes could probably go a long time before they even find out what the aliens look like, which can certainly be awesome. Also, you could all Plan Nine from Outer Space and have the aliens control dead people. Or, a bit stranger, it might only work on animals.
Possession - Similar to mind control, of course, but I'd categorize this as a more personal kind of domination: one alien possessor to one human victim. Maybe the aliens are biological creatures that physically enter or attach to their victims (à la Heinlein's puppet masters or the aliens from the War of the Worlds TV series),
Robots - Maybe the aliens are just self-directing, self-replicating machines. They could conceivably come in any number of purpose-designed forms, and destroying one physical unit might not harm the consciousness that's using it. Hell, all of the robots might have just one consciousness. Also, they might build bodies that can pass as human.
Tiny aliens - It might seem like a weird bit, but with enough craftiness and technology, tiny aliens could be a frightening threat. They might be be very difficult to spot or hit, certainly. Also, they could ride around in big, human-looking robots.
Titanic aliens - This could mean something like Cthulhu, the Celestials, or just the Zentradi. You wouldn't need all that many of them in order to make a perfectly viable invasion.
Alien superheroes - Like the (kind of hilarious) Shi'ar Imperial Guard, it makes sense that the aliens would have their own super-powered beings. Maybe they're the whole invasion force, or maybe they're just there to keep the heroes busy while the alien military does its work.
Divide and conquer - Nothing weakens your enemy more than getting them to fight amongst themselves. You could set up the whole invasion situation with some strife between terrestrial nations or super-powered factions that later on turns out to be orchestrated by the invaders.
Divide and conquer 2 - On the other hand, the aliens might openly court some human faction in a "When we take over, we'll put you in charge of this place" kind of deal.
Energy-based aliens - They might spiritual or psychic entities, ill-defined "cosmic energy" beings, sentient computer programs, or self-propagating patterns of electromagnetic energy. In all cases, of course, the obvious question will be "How in the hell do we kill something like this?"
Fluid aliens - Liquid-, gas-, or plasma-based creatures might have some of the immunities to attack that energy-based creatures would, but would at least be solid matter. Maybe they're shapeless masses that can only be destroyed by powerful energy attacks, or maybe they'd be semi-structures blobs that could only survive in extremely exotic environments.
Mineral-based aliens - That's right: ROCK MONSTERS. Not only would they naturally be insanely strong and resilient, but they could have weird, crystal-based technology.
Hive mind - A classic bit, of course: The aliens are all just parts of a single entity, all linked by telepathy or radio waves or whatever. They might seem insanely brave, as each one is just another body, easily sacrificed for the greater entity's goals. They might be able to incorporate human--and superhuman--bodies into their whole. They might, in fact, have no real unity of species, instead being made up of many different planets' dominated creatures.
Castes - Slightly connected to the hive mind bit, maybe the alien species is divided into specialized, physiologically distinct castes, ant style. The Bugs from Starship Troopers are a good example of this.
Virus - The aliens might deploy some kind of biological weapon, or they might actually be a dangerous microbe. Maybe the only "aliens" the heroes get to fight are infected Earth creatures, mutated and dominated by an alien infection. Naturally, the vector of transmission becomes incredibly important, here, as does the race to create a cure or vaccine. And, if a cure is possible, is it okay to kill infected humans?
Interdimensional invasion - There's no reason the aliens need to come from another planet, right? If instead they come from another dimension, they might be utterly bizarre beings that depend upon--and even bring with them--different physical laws . . . or, on the other hand, they might be humans (or hominids, or other Earth creatures) from a slightly or drastically different historical or evolutionary timeline.
Magic - Maybe the aliens are just sufficiently advanced, or maybe their technology really is some kind of fundamentally different willpower-based reality manipulation that we might as well call real magic. Maybe their actual physical gear (clothing, weapons, etc.) looks--and is--incredibly primitive, but their "magic" makes it possible for them to get to Earth and beat up on armies and superheroes.
Biological technology - This might be an overused bit, but it's still awesome. Living space ships and weapons, aliens that come in thousands of purpose-grown forms, the harvesting and implementation of superhuman DNA . . . lots of possibilities for cool stuff, here.
Energy-based technology - Kinda like magic, sure, but the trappings would be a little bit different. I'm picturing something like the divine weapons of Hindu myth, here.
The aliens are human! - Maybe they're a separate civilization of humans who left Earth (one way or another) thousands of years ago, maybe they're from another dimension, maybe there's some goofy panspermia explanation, or maybe it's just a completely unlikely coincidence.
Politeness - Sure, they have to invade Earth for whatever reason, but they're not assholes about it. They're very sorry about having to kill people, and they're perfectly willing to talk rationally about why it's completely necessary. Maybe they've even got honestly good reasons for what they're doing, which will just make things morally complicated for the heroes.
Factionalism - Humanity isn't a unified species by any stretch of the imagination, so why should we expect other intelligent races to be monolithic, undivided cultures? Maybe the invaders are just criminals or a rogue nations, and other aliens of their species want to protect us from them. Or maybe none of them care about us, but there are groups among them that hate each other far more than they hate us. Or maybe they're nominally united and in agreement about the invasion, but there are some fractures there are the PCs could go to work on, or a more rational faction they could try to make peace with.
Cultural influence - Like Baby Cakes says, "When the aliens come, they will be so great in so many different ways that everything we ever thought was cool will then make us ashamed." If the invasion is public knowledge and drags on for a while (and especially if it's less than completely violent), some people are going to start thinking the invaders are awesome.
Terraforming - If the aliens are planning on hanging around after the invasion (and if they don't really care about what happens to the locals), they might start by changing the place to be a little bit more like home. In fact, maybe the people of Earth won't even know they're being invaded until the sun starts getting smaller and the air smells a little bit like methane.
Life support gear - If Earth isn't perfectly to their liking, it's reasonable that the aliens will be running around with some kind of support system, from simple face masks or goggles, to straight-up space suits or environment tanks. Naturally, their life-support systems are vulnerable . . . but maybe they're actually dangerous to destroy due to the immense pressure, toxic gasses, strange energies, etc. they contain.