They could add Shards to VOG, to each boss that drops a weapon. So for instance the Templar would drop a shard of the Templar, having 3 of these would allow you to take a weapon that the Templar drops to the gunsmith to upgrade it's damage. 3 Shards for 1 upgrade slot but they SHOULDN'T be RNG, the Templar should drop one shard after each kill. That way with 3 characters you could upgraded 1 damage upgrade slot per week.