One of the things that keeps me coming back to Warhammer 40k is its variants. 40k is a big lovely mess of a game, and it supports so many kinds of play. Games that are technically better designed can't often do this, because they are so strongly about something.
Thus, I wanted to start a thread about 40k variants. Which ones have you played and which ones are you curious about? Which do you like the idea of, but think are broken, and how would you fix them. Let's go to town.
Personally, I have only played Cities of Death and Kill Team, but I am deeply curious about Planestrike and Spearhead. I'd also love to try Zone Mortalis. I'll write more later, but I don't want this to be a "listen to EP opine" thread, so I'll wait.