For example, only accounts that are at least 6 months old (or whatever age) can borrow cash. They can only borrow when a trade would be limited or rejected due to lack of funds.
Only accounts of a certain size can loan money out, and they have to have the cash available at the time of the loan.
Have lenders 'bid' for the loan, whoever offers the best rate gets the loan.
Have borrowers pay a large premium. For instance, if the lender offers it at 5%, the borrower actually pays 10% (per month or year or whatever), this would make borrowers think before going into debt, and keep it short term.
Actually, the rate should ridiculously high for borrowers. To discourage it.
Finally, the loan should be immediately and automatically paid back when the trade is closed. If the borrower creates a loss and can't repay, the next sell/cover in their account goes to pay it off, etc.
Come on, Antibody, just automate it, like trading is.
One big picture benefit of my idea is that it will get some excess cash out of the system. Just like comissions do now.