I would like to see Armor and Weapon repair linked to the respective crafting skills. Lets see it associated with a wide range of materials so that the demand for those mats keeps them flowing through the economy. If the gold sink is still required it is easy enough to put a fee on using workbenches and grindstones as well as the costs that are associated with buying materials for those who don't harvest them.
It would be great if this got incorporated directly into the trade window so that you were presented with an option to Repair players gear and could then post your price when you saw how much it would cost. The materials could be needed in the repairees inventory to simplify things. I suppose we would also want to keep access to npc repair vendors but that would have a set cost associated with the convenience, and it would still be good if the player was asked to provide materials for the vendor by having them in inventory as above.
The key thing would be those basic materials that we quickly outgrow as the population matures toward endgame lvl characters would continue to have some relevance if we all needed them for regular repairs. Our need for new gear begins to slow by midgame as lvl progression stretches out and people stop burning mats as they grind through crafting skills.
Need to repair? Go to marketplace and buy 10/10/10 of X/Y/Z. Go to nearby workbench and hit repair. Pay the associated fee and the needed mats are deducted from inventory.
Don't have that tradeskill? Hit up a vendor instead of the workbench or hit up a player standing within radius of workbench and the same thing happens.