Instead of using the expected return on the market portfolio and the risk-free rate in a CAPM

Question:

Instead of using the on the market and the risk-free rate in a CAPM approach to estimating the required return on equity, how would one use the firm's debt cost in a CAPM-type approach to estimate the firm's required return on equity?
Expected Return
The expected return is the profit or loss an investor anticipates on an investment that has known or anticipated rates of return (RoR). It is calculated by multiplying potential outcomes by the chances of them occurring and then totaling these... Portfolio
A portfolio is a grouping of financial assets such as stocks, bonds, commodities, currencies and cash equivalents, as well as their fund counterparts, including mutual, exchange-traded and closed funds. A portfolio can also consist of non-publicly...