Jonathan McMillan proposes a systemic solvency rule which stipulates that

[t]he value of the real assets of a company has to be greater than or equal to the value of the company’s liabilities in the worst financial state. (p. 147)

That is, the financial assets of a company have to be financed by equity. This reminds of Kotlikoff’s limited purpose banking, see here and here. McMillan (who is actually two persons, a banker and a journalist) argues that Kotlikoff’s proposal

is a step in the right direction to address the boundary problem, [but] it creates an overwhelming public authority [that monopolizes monitoring]. Moreover, it does not solve the boundary problem. Limited purpose banking requires the regulator to differentiate between financial and nonfinancial companies. … Finding clear legal criteria to categorize a company as financial is impossible. (p. 140)