I have an idea for how to implement it too. Reforge requires 3 blue weapons or 1 legendary of the same category, being primary special or heavy. This way there is still a grind involved and returning to town will grant a smaller number of chances at reroll. I would personally prefer to be able to craft the perks I wanted on a weapon though, and have to farm a variety of materials, some dropping from strike bosses. I mean you gain "experience" with weapons, the perks should be chosen after you've determined what your weapon needs to be better. The idea is that you're literally making attachments and modifications to the weapon anyways, why would anyone mod a weapon for something they don't want?