I would recommend not making it an mmo. Tough to program, servers would be real expensive, constantly have to put out tons of content (not easy if you want it to be quality stuff), will probably be irrelevant in a year with too little for hardcore/too much for new players, only one where you can save the world doesn't work when 5000 people are also the only one, story's are easier to write for single/co-op games as you get to control the pace. I would also limit the amount of weapons the player can use( be it one of each weapon class or take out some weapon types altogether). Less is more in a lot of cases. Having too many choices may seem like a good thing, but if not done properly, could easily overwhelm the player and maybe make them quit.