To pile on one more suggestion: Part of the customization could be that perks are locked behind certain challenge sets, like the thorn quest. For example you get the "frame" of an fn herstal 5-7 (sidearm) and several quest lines are offered to you, each of which grants the weapon a particular elemental damage type. To complete these however would require the player to complete multiple challenges. Afterwards the next perk is selected via the same route. That way the player could build a custom weapon by selecting the perks they want, but would have to put in considerable work to do so. Idea for one of the quest lines: "a cold and bitter war," would grant solar damage or full auto. Task 1 = capture 500 control points, task 2 = destabilize the fallen and give occupation of the moon by drawing a large contingent of hive forces (max level) onto the surface (think Indiana Jones boulder scene), task 3 = amped up Valus strike with void burn (think that's the right element) and extra major scions where named ones have to be taken out before the boss, final task = VoG hard without a wipe. Should also have new perks like the ability to steal ammo with every shot that lands etc, crazy stuff but would require crazy time investment.