I have often found myself in situations where I really want to purchase a particular mod, but it often can take me a long time to find anyone that has the one I want, at a reasonable price. I'm sure I'm not the only one who has found himself in such a situation. So this brings me along to my suggestion:
What I would really like to see in GW is a significant addition to the weapon merchant. Not only should you be able to customize your current weapons and craft new ones, it'd be fantastic if they could add mods to your weapons too. The amount each mod costs could be proportionate to how strong it is (ie. a staff wrapping of enchantment that offers 20% longer enchantments could cost like 20,000gp, whereas a staff wrapping of enchanting that only offers 12% longer enchantments could go for about 12,000gp). Additionally to keep things fair, make it so that getting mods in this way, as with runes when you get them from the rune merchant, does not unlock anything.
My apologies if this has been suggested before (I used the search feature and found nothing).
EDIT: A minor addition to the mod power and price scaling idea. Perhaps make it that earlier weapon merchants in places like ascalon can only offer only up to 1/4 of the max mod. As you progress through towns, while the weapons that are available for crafting get better, so too do the mods.
A


So instead of wasting 2 hours trying to find someone who would actually keep a +24, I could simply walk to the trader and purchase it. This wouldn't cause any imbalance in the economy, as stated before traders still reflect supply and demand.