Main purpose being lending of loan to nations for interest.
Interest should be depending on factors like: Number of countris the lendee is in war with. Higher the count, higher the interest.
Lending between nations, with automatic repayments and in case of defaults province should be transfered ( all automated) terms of which will be agreed upon at the time of lending.
Most powerful weapon, which can destroy one whole province. But development of which should be too costly and secretive. Like if you have doubt that any nation is developing such weapon , we can deploy Spies and discover that. And also slow the process down.