I suggest Safety Deposit Boxes be added to the city bank.
Make them buyable/rentable with points. Most banks charge a monthly fee for use, so that could be a point sink ( barely ).
These boxes could allow players to have more liquid cash accessible. Boxes could hold anywhere up to something like 1B. On top of the Private Island Vault.
This allows a little more flexibility for players who tend to be afraid of the stock market for cash holdings.
Accessible whenever the player needs.
Good idea? Bad idea? Tweeks?