Technically, this is completely fulfilled by the fact you can change your job/class on the same character. Unless what you're getting at is something along the lines of a PLD using a bow to tank with... which just seems odd lol. Or even under the idea of role changing based on it, you already have that option, as I indicated on the first sentence. Unlike other games, your character, through changing weapons here (effectively changing your role/job/class), is actually doing exactly what you're thinking of.

The idea for multiple weapons could be interesting, but it'd certainly have to be VERY limited. This game, unlike others like WoW, is more about the flashiness of it than otherwise. The way this game is set up to play (slower/fluid action animation and combat than things like WoW) wouldn't allow for such options in the near future. Classes themselves would have to be altered, both abilities and stats, which really is just not going to happen here. FFXI also had things like this, and while it was fine at first, it kinda just came boiling down to a specific weapon type per job.