You mean like 3rd and part of 4th edition?
There were tons of variants for basically all of the armies
Sure, the Marines had their half dozen or whatever books, but Chapter Approved added tons more options for the Marines and almost everyone else.
Tyranid Seeding Swarms, Feral Orks, Kroot Mercs, Chosen of Ahriman, Armored Companies, Swamp Warfare rules, Ork Klanz and much, much more.
We need to make a return to those times... it's the things like the new Eldar Corsairs army list that keep me at all interested in the hobby these days.