Anything where the underlying math hasn't been proven at Amazon/Apple/Google scale, and/or isn't available in production-ready form in open-source software, and/or is heavily patented.
Just a couple days ago, I heard a mathematician call a certain complicated knot invariant "space math" because of how out there the approach was, but that was idiosyncratic.
Presumably whiz-bang space-age cryptostuff, homomorphic encryption and differential privacy et. al.. Things that'd be spectacular if you found them in a startup two years from now, much less in an enterprise that hasn't done any core tech upgrades since 2005.
