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Bulletstorm happens in the same universe as Borderlands.The events of Bulletstorm happen years before those of Borderlands. The charged weapon effects of Bulletstorm are prototypes of the full-time ones in Borderlands. The leash is an early form of a combat ability or classmod. Personal shields aren't in use yet. Stygia, as a planet, later becomes a testbed for weapons used on Pandora; their effects on both degenerate humans and alien lifeforms well understood by now. The dropkit system, and its instantaneous delivery system, is later privatized and commercialized into vending machines, which use money instead of skillpoints to provide services. Skillpoints themselves have been folded into a more practical experience point system based on number of kills as opposed to style in those kills, mainly to discourage insane combat stunts, and encourage a more practical approach to combat. Hell, it's possible that Mordecai's Bloodwing is native wildlife to Stygia.
Alternatively, Bulletstorm takes place in the same Universe as Gears of War.Sera could be a long lost colony of the Confederacy. Future stories could show the Seran humans reestablishing contact with the wider galaxy.