- Add taxes based on how many vehicles and houses you own. It shouldn't be that harsh at first but it should really have an impact on your bank account if you are too wealthy.
- Winning prizes for all casino games and horse races should be way lower.
- Increase the ammo prices.
- Increase the /supply prices.
- Increase the car upgrades prices.
- Add more houses to SF/Countryside/Desert, especially SF as there's many unused apartments. I know this has been suggested 100 times before but I am just reminding it. This would greatly increase the "property market" and it will make people do something with their unused money.
Basically, if you are wealthy as hell, you should really work for your money if you want to keep it that way. Any more ideas will be welcome.