MacTeX 2019 doesn't appear to be available yet and Homebrew isn't sufficiently wise to work with pre-releases in-advance to get formulas working sooner rather than later.
What are you talking about? MacTeX is already available [1]. MacTex is cask-only on Homebrew, so there's really not much to test in the formula. You should be able to bump up the version in the old cask formula and it should work. Having said that, installing from a cask doesn't make much sense for a program that releases a new version once per year.
