Build your own weapon? How about a bounty where you can choose what weapon type. And the perks( from a set). Of course there would be the same "go to this person and get that thing" and "kill lots of stuff". But each guardian can choose what type of weapon, say a auto rifle, and work through the bounty to collect perks. Maybe at one stage there are three option for perks and each has its own requirements. Say one perk is hip fire which requires 500 PvP precision kills with hip fire. Or say you want it to have solar damage, yep. 500 PvP solar kills without dying. Not trying to say it all has to be PvP. Just what is coming to mind. Could be drops from fallen with a low drop rate. Or a engram conversion to something chance. What do you think.