You don't learn things from flash cards, you fix things you have already learned into your memory. And memorizing the formulas encoding theorems is definitely helpful.

Pearl's causality stuff and Jaynes' Bayes stuff are probably something a lot of LW readers want to know better. I don't know how ankifiable they are, I have a very poor grasp of either. My impression is that probability math is a bit more loaded with hard to formalize background concepts (cf. Tim Gowers' musings about just what random variables are) than some other fields. This might make a straightforward flashcard approach less useful.

Category theory and abstract algebra seem to start with a bunch of basic definitions which you could memorize with flashcards, but getting anywhere in them even with the definitions handily uploaded in your brain takes quite a bit of mathematical maturity. I tried to do CT with Anki, but got bored quickly since I didn't have any study plan beyond that and am nowhere near the mathematical skill where I could just start doing interesting stuff by myself using the definitions.

Not really, I thought you would like general input on the possible topics.