1) Modding can help bring more money into the game. If you implement a store which sells mods it not only can make more money for Bethesda, it can encourage more people to play the game. You'll get people coming that might not like a visual aspect of the game, or maybe want a new armor or skin added.
- You can further encourage people to submit new mods and skins by giving them points to use on the market, points that can also perhaps use towards buying more time on their subscription in case they don't want to buy mods or anything else on the market. They could earn a comission for each purchase of their upload, example: A new sword with decent stats goes on the market for 150 points, these points bought on a Bethesda website and (or) earned as part of a subscription fee. The person who created this mod could earn ten percent for each download, 15 points for each download which can be given each night at a set period, like servers being given maintenance.
2) Sometimes people just don't like who a species looks, most commonly I've seen reskins of Elves but sometimes Argonians and Khajiit have been reskinned for games as well and these new skins can look -incredible-, and they only encourage people to play more because their characters and those around them will be much more improved.
-I wouldn't think this would actually be as hard to implement as some might say. People have been altering skins in MMO's for years, nvde patches being a prime example. Of course, I'm not suggesting this be something sold of the market, but it wouldn't hurt.
3) Modding makes people feel even closer to the game and its universe. I once made a mod for a Bethesda game and I felt great helping even a small amount of people to enjoy the game more, it also gave me a deeper appreciation for the game.
4) This market could also be a great place for Bethesda to sell content they develop themselves. Microtransactions and F2P are the future of MMO's, a market like this would give them a leg up if and when they decide to go F2P. (I've read this will be a subscription based game, forgive me if I'm wrong.) Having a market like this set up early on would give them time to perfect the system and make it a viable alternative to subscriptions.
Anyways, those are my thoughts. What do you all think?