My Multiworlds setting had Earth being attacked circa 2015. The backstory was that there was a massive war going on between alien species over a certain mineral. The mineral was highly useful, but found on only one planet. Earth has nothing to do with the mineral. In the course of the massive war that broke out, a superweapon got accidentally released and the planet was quite unfortunately blown into tiny bits, armies, civilians and all. Realising their mistake (but not quite forgiving each other), the four races arranged a Treaty that was essentially an agreement for controlling the escalation of battle. Both sides fought with as little tech as they could, and once a piece of technology was used against a side in a given battle, that side had the right to use this technology as well, without repercussions. While not actually limiting, the Treaty served a good reminder about the committed atrocity, and the sides followed it for quite a while. Fast forward to 2015. A small assault fleet of one race is being chased to the edge of the galaxy by a large hunting force of another. The fleets still follow the Treaty, preferring boarding teams to onboard weapons for fighting. Seeing no alternative, the assault fleet opens fire on the ships chasing it, thus winning itself some time, and hides its signatures by approaching our planet with all the electronic noise it emits. A miscommunication later, alien attack craft rain death on Earth cities and Russian nukes decimate the assault fleet. Russia is turned into a radioactive wasteland (mostly due to fallout), most of the world's industry is geared towards repairing the damaged, and building new ships for the aliens to use. Earth military, what remains of it, uses USA's (better developed than currently) missile defence network to mostly fend off the aliens from its territory. Some years pass like this, until in 2025 a Resistance movement manages to gain control of all the aliens' installations on Earth. Just as the humans take over everything, four fleets from the different races arrive, following the traces left by the assault fleet (knocking out the communications network was not a good idea), and a fight begins for Earth. Two of the races recognize the humans' rights and introduce them to the Treaty. The combined forces persuade the other two to follow the Treaty as well.
The story then branches off into the Supernatural and Cliche (well, more cliche). It turns out a piece of the mineral ore from the original planet did end up on Earth. One thing after another, a mysterious all-powerful person appears amidst combat, and sees that the fight is quite unbalanced against us Humans. The all-powerful person then enforces the Treaty, by dividing the piece of Mineral ore between representatives of all five races, that become Battle Controllers in charge of enforcing this Treaty. The human representative gets a bigger share of the mineral, of course, and becomes the one whose name I use on the net - Controller Sean Mirrsen. It all goes downhill from there.
I'm not really sure how well it would work for an FPS. It's better suited for an RTS, but provides a good selection of both normal projectile weapons, energy weapons, melee weapons, and aliens. The original aliens weren't too original - two kinds of humanoids with animal features, a different take on the Grays, and some cyborgs. The first part of the game could be about the Resistance getting its act together, then fighting aliens on equal terms, then the Treaty and whatever ending you can think up to the thing.