• 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.

Update: And now, things I've recently learned