Ultra-TeX mode http://www.math.washington.edu/~palmieri/Emacs/ultratex.html is a major mode for typing TeX documents written by JohnPalmieri. One of its main features is dynamic completion through LightningCompletion on TeX commands.

The math.washington.edu page seems to be gone, but JohnPalmieri’s homepage (http://www.math.washington.edu/~palmieri/programs.php) now links to a GitHub repo: https://github.com/jhpalmieri/ultratex

Even though the readme claims it’s an old version, the files say it’s version 0.8.