Honestly I have hard time imagining how would you possibly create Modular RPG game kit.
RPGs are so interconnected systems, where one thing can affect two others (for simple reason of properly implementing RPG mechanics), that I don’t really see it work. Unless by RPG you understand well, we have levels, we have quests and we have two characters stats. Then whatever it might be possible ;p.
My point is that even simple sword swing or shoot from gun, will be depedant on some other data coming from different source (like character stats, buffs, debuffs, etc), and you must take care about this data at weapon level (in one form or another), to make sure it properly affecting you actions.