Programming language theory and formal verification have been relatively hot during the last 10-15 years and show no signs of slowdown. Still, a relatively niche area.
Also the intersection of CS, probability and statistics is a very interesting area to work on. Less trendy than deep ML, but really practical. See e.g. Stan, Pyro, Andrew Gelman's books, etc.
Thanks for the insight. My Software Quality prof gifted me a copy of one of Gelman's texts but I haven't had time to take it in; I should change that...
It's weird to me that formal verification isn't more widely used; I would think it would be common at least in safety critical systems development.
Also the intersection of CS, probability and statistics is a very interesting area to work on. Less trendy than deep ML, but really practical. See e.g. Stan, Pyro, Andrew Gelman's books, etc.