It would give the gunsmith some business, and give us something worthwhile to use our Motes for.
Also would give me a reason to have more than one type of the same gun. I usually just dismantle ones with worse stats to save on space. It would be fun having an Ice Breaker of each element.
Edit: Legendary weapons only....ignore my greedy Ice Breaker comment.
Edit 2: Maybe make it just one reforge session per weapon..once you leave the vendor after reforging, that gun can't be reforged again.
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I think they are using the iron banner to test the feature for later deployment in blacksmith and vendors