It's already been shown that Bungie has the ability to upgrade damage rates on weapons (Mythoclast from 300-323) so I'm sure it'll be possible for them to allow players to upgrade their own weapons too.
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It's not a question of "can they" It's a question of "will they?" The Mythocast isn't a good example. sure they proved they can raise it's attack... but it's due to feedback about the original nerf, nerf it too hard. I wouldn't mind so much if.. it's just ultimate-raid-rewards like the Mythocast are the only weapons that get higher than 300, but not when our favorite non-exotic guns get left behind because of a single value.