So I finished the main game with Plasma Pak, and I’ve moved on to Cryptic Passage. While this wasn’t made by Monolith(*) it is generally considered an essential part of the Blood universe.
As it was made and published by external parties there are no added features; no new weapons, enemies or items. What you do get are several large (at least they feel that way to me) levels of imaginative and inspired design, along with new complementary textures.
If you’re finished with the main game, give CP a go. Even if you haven’t finished it, it may still be worth trying CP before the monthly challenge is over.
(*) The company who did make CP was Sunstorm Interactive, who also created a number of addons for BUILD games, and would later make the Deer Hunter series.