Idea 1:
Regardless whether you find it easy to make money or not, many Glitches have loads and loads of it. It would provide a lot of end-game incentive if there was an item-line developed that perhaps satirized actual real-life ridiculous things you could buy, but was still extremely highly priced. House expansions could be something that would fit this, if there was an uber-tier level of expansions for the super, super rich. $100,000+ in price, or something similar. If the items were actually useful or seemingly of great value then this would really stimulate the whole in-game economy, as more players would want more money.
Idea 2:
A way to take out loans ranging from $10,000 - $100,000+. The higher level loans might only be able to accessed by higher leveled players. If you don't pay back your loans, you end up with debilitating permanent effects to your character. If it was run by the game, it would be extremely legitimate and no one would be sketched out about utilizing it. There could be a savings/interest system, as well.