Thanks for your varied suggestions!

Actually I'm kind of more comfortable with MIRI math than with ML math, but the research group here is more interested in machine learning. If I recommended them to look into provability logic, they would get big eyes and say Whoa!, but no more. If, however, I do ML research in the direction of AI safety, they would get interested. (And they are getting interested, but (1) they can't switch their research too quickly and (2) I don't know enough Japanese and the students don't know enough English to make any kind of lunchtime or hallway conversation about AI safety possible.)