For those with PHDs or of some high math credentials: What would be the next subject to learn in math and why? After having taken these, what's next? Probabi

For those with PHDs or of some high math credentials: What would be the next subject to learn in math and why? After having taken these, what's next? Probability Theory, Calc Series, Linear Algebra / Differential Eqs / Discrete Math. Could you annotate something about the subject with an example?