If one were to construe a systemic risk that could bring bank systems down, this is one way

First: Make capital requirements for banks based on perceived risk. More risk more capital, less risk less capital. That would allow banks to leverage more with The Safe than with The Risky. That would allow banks to earn higher risk adjusted returns on equity lending to The Safe than when lending to The Risky.

Second: Allow banks to use their own risk models to decide what is risky and what is safe and therefore how much capital it needs. Alternatively allow some very few human fallible credit rating agencies to decide what is safe and what is risky.

Third: Sit down and wait for banks lending too much against too little capital to The Safe, like sovereigns, the AAArisktocracy and mortgages; within an economy weakened by too little lending to The Risky, like to SMEs and entrepreneurs.

But, oops, hold it there! Someone already did that! I think it was the Basel Committee for Banking Supervision.