Well, the only idea I got is that Debt needs to actually have an impact on a nation. Allow a "Collateral" Casus Belli. Assign a fixed $$$ value to a province/state according to Industry, Overall income of the population and troops recruited from that region. LIMIT how much of a loan you can get from your own banks, unless you allow Capitalists to actually OWN Factories and troops and provinces as some AI-ran satelite nation
Example: As the USA, you are several hundreds of thousands in debt on a loan given to you by the Bank of the USA. Unable to pay, your allowed to default and thus "remove" the loan by seceding some province to a Capitalist-based "AI Satelite Nation" of which you can't declare war on ever, but you have the option of one day buying back what they own. This Capitalist-based AI Satelite Nation can, after a fixed amount of years, become "On the market" but at a very greatly increased amount (A property that was sold to cover a 100,000 debt, can be bought only for 100,000,000). However, this Capitalist-based AI Sat. Nation (The Corpocratic States of America, the United Corpocratic Kingdom, Corpocratic Canada) is War-Immune, operates on a basis of a fixed amount (15%) of the pop as capitalists, and has a production modifier to it. You CAN put it in your Sphere, but to get it back or take it from another, requires money.
Many ways to approach it, but debt needs to have an actual impact instead of just a laughable small amount of debt.