I prefer Oldhammer to modern 40k tonally, so have created shall we think of it as a fork from the 40k universe. The conceit is a sector that has been cut off from the outside galaxy by constant almost nearly impenetrable warp storms for the last several thousand years.
Within this bubble my personal Space Marine and Imperial Guard forces exist, as do Ork forces run by a friend. They have history and fluff. I make fluff for the planets and major figures in this sector, creating a personal canon with 40k’s canon being a foundation that I feel comfortable being loose with.
So far most actual gaming has been done with the OnePageRules systems, though I am trying to drum up interest in playing 2e or 3e games for novelty.