Broncosaurus Rex, from Goodman Games, might be a good option for you. That one mixes cowboys and dinosaurs, of course, but it is a good start. It's also a science-fantasy setting, with starships and all, but you could change that to magical portals or whatever variant you want. Maybe even just have Cretasus, which is the dinosaur world, be just a continent on a planet where the Confederacy and the Union are on another continent...
Add in a new faction on the planet, one from a Japanese style setting, and you've got your samurai. This could be a whole stellar empire, like the Kurita from BattleTech, or another continent on the planet, if you are going with that option, or an empire on a whole different world if you are going with the portals option.
Barbarians are easy... they could be local "neanderthal" style tribesmen, "lost world" tribes like out of Burroughs or Carter, or even wandering Scotsmen from yet another continent or world...
You can then add in Magical Mechs if you want, using Goodman Games' DragonMech...
To wrap it all up in a more fantasy than science setting, dig up the DragonStar setting from Fantasy Flight Games. That sets the planet in a fantasy-science galactic-spanning civilization. You've got the Confederacy and the Union from outside Dragonspace, who have settled Cretasus on the borders of the Draconic Star Empire with their cowboys and soldiers. To preserve their claim, the Dragons invade part of the planet, primarily with their warriors and monks from one of their own Oriental style worlds from nearby. Into the mix of this, wanders some Space Barbarians, clad in kilts, and weilding broadswords, riding the backs of giant space whales, and manning their ancestral DragonMechs. Of course, there are the dinosarus of the planet themselves, who just wanna be left alone...
And you can add in the races from Red Steel, too... Lupines (dog people), Rakasta (cat people), Tortles (tortle people), and Aranea (spider people). Heck, you could have some teenage monkish ninja tortles, even...
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