Dr. G, while those are interesting ideas, I think the major flaw with them all is that the only reason to bet or invest is to make more money which is what you are trying to use in the first place. Taxes would just be a pain in the [censored] with no benefit besides draining off your profits.
Yes, the idea of betting and investing have the possibility to make a profit, there is also the possibility of getting nothing in return. Betting on Gladitorial combat would be something for the lower income people to play around with, by spending 100 septims here, 500 septims there, but then you can get into the higher echelons of the investments. Lets say X wants to start a business, but needs 25,000 septims to set up a shop in Y town. You can either choose to pay the person the money and have hopes they will achieve their goal, set up shop, and you would get whatever back, such as a unique item, septims, free gear, or losing all your money. It would just be a sideshow attraction, but could be fun.