I have several suggestions:
1. Inter-bank lending. Banks can lend/borrow money to/from other banks with relatively low interest rate, which is quite common in real world setting.
2. Enable banks to invest bond, which is also quite common.
3. Competitive lending interest rate.
4. Pure e-commerce company. Companies such as Amazon and Alibaba manufacture nothing, they just have a platform and other sell goods on their platform. I wonder if we could make this happen in the game.