More datastructures
Especially more functional datastructures
Deeper understanding of monads
The propagator model
More on operating system theory
Deeper understanding of crypto math
More about compilers
The propagator model
Neural networks and other machine learning techniques (related to the above)
Would be great to try my hand at implementing capabilities
More on the bottomless well of type theory
Haskell and the ability to overcome my difficulty reading that syntax (and kind of related syntaxes, like ML)
Theorem proving. Start The Little Prover!
Logic programming. Finish The Reasoned Schemer!

I'm sure there's things missing from that list that follow the "gee, I wish I knew more about these things" pattern.