You should try Dungeon World. The system is way less restrictive than D&D and you’ll be able to add firearms and even turrets just by thinking about it. All the goodies of GW2 are really easy to translate for this game because the game is really meant for fun and action oriented gameplay. The system is really modular and fast. I read the books last week and I could GM a game after reading the first couple of pages of a free PDF on their site. It basically answers all the questions I had like : What? No initiative? How am I suppose to manage combat? Etc..