I'd prefer not to have any designated weapon slots since it could separate the weapon sandbox into multiple classes, thus making some weapons better than others instead of making them both useful in different ways. This would really restrict player choices in a game that wants you to make gut decisions, like with picking a race. For example, a player could be turned off from PvP because he loves Pistols, a weapon that is inferior to the "big guns." That player would then be forced to adopt a new style, taking away the fun of the game. The weapons also don't have to be balanced in a traditional way, like "smaller mag = more damage," but in a way that allows the player to benefit from its play style. For example, a Pistol could have a weaker round than an Assault Rifle (because that makes sense) and do less damage and have less rang, but have other perks such as faster reloads and faster movement, making it better for close quarters. The Pistol could also be upgraded over time, giving it more damage, better range/accuracy, and bigger mags. But to answer your question, I think that 2 undesignated slots would be the best for a game like Destiny, but you should also be able to store a few weapons on back up that you could swap out (this would take a longer time than just pressing Y).