If you're looking for class/archetype ideas, then one that comes to mind would be the plucky engineer or tinker, who makes his own crossbow, and upgrades it in various ways so that it shoots more bolts, faster, and makes specialized bolts that do crazy stuff like explode, spit acid, create a sonic boom to deafen people, latch onto walls and make a grappling hook, etc, etc.

Crossbows don't have much of an established mythology since they're the weapon of the common man, and weren't around long enough or sufficiently different from bows enough to make their own legends, at least none that are common knowledge today. Instead, you're better off either tapping into the scientist/engineer mythos, or possibly some of the early gun mythos, IE a gunslinger type crossbow user using hand crossbows as revolver proxies. The demon hunter from Diablo 3 is a good example of the latter.