These are all great thoughts, I appreciate everyone's input!
I think scepter could be an interesting addition to thief, as long as it's mid-ranged and not something far-reaching like Guardian scepter. If it's to do with close-ranged magic, or something that does local AoE, I think it'd fit with other thief playstyles, but just with more survivability and area denial, perhaps.
As for Luxon cannons, I think we're all on a similar page here. However, I'd like to think that between Kurzick understanding of what is essentially Canthan-Druid magic, and Luxon Siege Turtles, Cantha would have the foundation required to develop some form of hand-held projectile, magic or not.