I think it would be pretty sweet if some weapons and armor (maybe legendary or exotic) had blank upgrade slots. You would unlock the use of these blank slots just like other upgrades (defense, more ammo, weapon stability, etc.) but you could attach an upgrade of your choosing to this slot. These custom upgrades could be sold at the crucible armory, or by factions. I think this would be beneficial, especially since there is no trading (which I do think is a good thing). 'This is my rifle ...'