Purchase a Business - This would be very expensive but it would give a player a goal and a small return and of course they would be run by NPC's. Types of business that could be purchasable options could include the following:
Lumber Mills
Tavern/Inns
Farms
Merchant Shops
Goldenglow Estates
You could create small monopolies, for example if you own all of the lumber mills in Skyrim you would get a small bump in profit since there is no competition! Lets not forget the merchant wars of trying to run other merchants out of business, sending assassins and hired thugs to send a message, and sabotaging business.
The Merchant Guild could offer tones of quest and investment opportunities, a banking system with the option to purchase gold and silver ingots. There could be a Merchant Guild vault where you could store coins, gold, and silver ingots and make a small profit in interest. There could be the option to invest in trading ventures high and low risk, obliviously high risk investments would have a higher profit but your chance to lose your investment would be higher. Maybe the option to buy your own trading/cargo ship! Imagine becoming the owner of East Empire Trading Company which could be the final quest for the Merchants Guild I mean somebody has to break up that monopoly and get a piece of that action! Lets not forget some more black market options like Balmora Blue and Skooma smuggling. I guess I can dream for a Merchant Guild dlc, it could even expand the Thieves Guild a bit by adding followers and hireable mercenaries. Anyways I was just thinking of how I would like to see gameplay improved for higher leveled characters with more economic options to spending all of that loot. What are you guys thoughts?