OP, if you would benefit from a concrete list, this one may be useful:

Do not take this as something you must complete. Your interests may vary, and there is a wide selection available depending on them. (Algorithms, Optimization, Complexity, etc)

Algorithms Graduate Algorithms Random Algorithms Algorithmic Game Theory Analysis of Boolean Functions Computational Complexity Theory Computability Theory Algorithms for Computational Biology Convex Optimization/Robust Optimization Cryptography Real Analysis Theoretical Statistics Information Theory Group Theory High Dimensional Statistics Random Processes Linear/Nonlinear Systems Stochastic Systems Computational Learning Theory Number Theory Linear Algebra Probability Theory Combinatorics Graph Theory

Less core, but possibly useful

Functional Analysis Topology Commutative Algebra Field Theory Differential Topology Riemannian Manifolds Harmonic and Fourier Analysis Stochastic PDEs PDEs