A lot of banks and brokerages will allow customers to borrow cash against their investments. These loans can be very competitive and dynamic: their interest is computed daily and can be marked to specific benchmark rates with very competitive markups (e.g. InteractiveBrokers offers Fed+1% loans, which today comes up at 1.33%).

The size of these loans moves daily based on the market value of the underlying assets, but can be approximated with a fixed %. E.g. if I have 1M$ in low risk/volatility assets, IBKR might allow me to withdraw up to 50% of that (500k$).

A lot of wealthy folks use these sort of margin accounts as checking accounts, in lieu of selling their investments, so that they never need to realize a capital gain on their investment (they just keep borrowing more and more as their investment grows).

Being able to model such an account would be very useful, as it can prove to be an invaluable tool. It acts sort of like a credit line with a variable interest rate and a variable maximum. The following features would suffice:

• Be able to model benchmark interest rates (not currently available, but related to inflation).
• Have a debt that has interest computed on a rate that moves with benchmark interest rates.
• Have this debt be usable for withdrawals in cash-flow priorities.
• Have the debt ceiling grow as a % of investments.