I was thinking that it might be helpful for players to have some form of a weapon fusion system similar to Valence Fusion in Warframe. The general premise with Valence Fusion is that with specific weapons or Railjack (ship) parts you can fuse them to improve their stats but only up to a certain point.

For Black Desert I figured being able to fuse e.g. two Kzarka weapons up to a particular point in the enhancement scale or being able to convert spare boss weapons/armor directly into Caphras levels might be a welcome addition and pathway to gear progression. I can also imagine that it might push players towards content that yields these weapons/armors more. Naturally there would need to be some form of balancing here but I think alternative paths to progression are worth looking at at least.

Like? You can just sell your excess boss stuff on market, and buy chapras on the market?

That's literately why market exits in the first place. It allow goods to be exchanged between people, so if someone have excess he can trade it for other stuff.