I write Yet Another Alternate Heresy, the Vulkanite Heresy, which can be found on Archive of our Own. Perturabo And Mortarion Did Nothing Wrong.
I also make homebrew fluff for various chapters and warbands. My two ongoing series are Tzeentchian warbands which aren't Thousand Sons descendants. and successor chapters to First Founding chapters who go significantly against type; Blood Angels descendants who have removed the Black Rage (by essentially turning those who fall into it into Blood Rubricae), Raven Guard descendants who think the Sable Brand is awesome, etc.
I also do crunch. Index - Corsairs is my work, as is most of Codex - Rogue Traders: /tg/'s 8th Edition. The rest is gathered at User:Auroch/HomebrewCrunch. It includes stats for Sisters Famulous I suggested for Codex - Adeptus Ministorum /tg/ Edition and some rules for Ursus Claws on big Knights and small Titans. Uploaded eventually will be some rules for making Death Guard Plague Companies play differently (a la Thousand Sons Cults) and rules for putting the Chaos in Chaos Knights, aligning your warband of Knights with one of the Big Four. The Vulkanite Heresy, especially Eldar, also have some new crunch I will someday upload.
The most detailed crunch I have is silicon-based-lifeform Space Dorfs called the Durazi. Highlights include DORF REHN, Troops more durable than some tanks, and glacially slow movement. Fastest infantry in the Codex has M 5" and advances 3+D6"; second fastest has M 5" and advances D3"; everyone else is slower. On the other hand, those are also the only units in the Codex without Invuln 6+ or better, every infantry has Feel No Pain 5+, and every vehicle (except the disposable one used for DORF REHN) regains a wound per turn. Oh, and everything has names in both English and Khazalid/"Durzalid", courtesy of an amateur linguist who prefers to keep his username far away from things 40k from fear of SJW bullies. Which is fair, I guess.
Random Tables are great, but annoying to write without a visual editor. Therefore I start on Wikipedia and copy here; sandbox