I'd love to see the Arc6 being "expanded" into a sort of Arc LS / TnC-like Lego system.
Different sleeves (CR123, 18650, 14500, 17670, CR2), additional heads (larger, throwier head, mule type head, long-run/high-efficiency head), different pistons (w/ different plungers), perhaps even a few compact, pistonless (twisty-only) bodies.
I know it's never going to happen, but hey... i can dream :D
PS: A more high-techy, high-output / high-dynamic range AAA light would be nice too! Sorta like a cross between an Arc6 and an Arc-AAA.