Burn should be separate from the gun IMO. Burns were removed from primaries because it was felt that they killed weapon choice and weapon variety. I disagree, but to the extent that choices were rather limited, it was not because elemental primaries existed - it is because there were to few weapons with a burn. The solution is elemental cores which drop from completing endgame activities (I would propose completing the hardmode raid). These cores can be used to add a permanent burn to ANY weapon you want. This does two things: 1) Allows the player to customize their weapon selections and remold their weapons to fit certain combat roles for certain activities, thereby giving us an incentive to change weapons based on combat situation and burn; 2) It gives us a reason to repeatedly replay endgame content, because you will want a number of elemental cores, not just 1 or 3. This is what should have happened when TTK launched, but instead, boneheaded Luke Smith happened instead.