Dinosaurs (from Earth) from Space!
Alternate prehistory: The dinosaurs were not wiped out by a comet, but an environmental catastrophe of their own making, possibly caused by weapons of mass destruction. (Some species, such as velociraptor, were sapient and civilized.) The wealthiest and most politically powerful among them escaped earth in spaceships (bringing along some of their servants and enormous brutish pets) and have since been zooming around in circles at near-lightspeed, taking advantage of time dilation to give earth time to be habitable again.
Dino scouts monitored the earth from then up to and including all of human history, occasionally being spotted in their flying saucer-type ships. They were at first not particularly concerned (knowing they could wipe out humanity in a heartbeat) but starting in 1945 with the dropping of the atomic bomb they began to worry that humans would destroy the earth again, so they set course to return, a few million years earlier than planned.
In 2015 they finally arrived and launched a full-on assault, easily claiming the earth due to their superior strength and agility (not to mention their advanced weaponry). They killed most of the human race and enslaved what remained.
The new society is highly stratified, layering the dinos' existing species/caste class system on top of the newly-enslaved humans. (The dinosaur society is a thinly-veiled metaphor for wealth stratification in the real world.)
The heroes (a ragtag band of survivors) must reclaim earth for its rightful inheritors, while learning valuable lessons about environmental stewardship and the dangers of unchecked power.
I picture this with a Saturday morning cartoon vibe, but it could just as easily be one of those action movies that straddles the fence between gritty and silly.
If I ran it as an RPG campaign, I'd use Fate Core, with the optional weapon and armor ratings to emphasize power disparity. (An ordinary gun would be Weapon:1—not too effective against, say, a T-Rex!) It would also work well in Savage Worlds or an OSR system like Stars Without Number/Other Dust. Some people might want to play as rebel dinos… and I'd definitely let them, because that would be awesome! I'd start the campaign a few months after the dino invasion.
Adventures might involve delivering messages, liberating communities, raising a guerrilla army, capturing key strategic points (e.g. air defense turrets), gathering firepower, investigating earth's ancient past, taking out dinosaur VIPs, and ultimately, capturing or destroying the huge dino mothership that hangs visibly in orbit.
There are any number of possible outcomes to this campaign: complete dinocide, humans and dinos coexisting in harmony, dinos fleeing in their spaceships to seek a new home planet, dinosaurs enslaved by humans, and so on.
This could also be pretty cool as a video game—maybe an old-school JRPG.