I was just walking down the hall here, and trying to decide on my next project, or at least what Im going to tell myself my next project will be.
I want to do something with Space Marines (I know, big surprise!), but not standard Imperials. Ive got a trail of armies already from that line. Chaos is more tempting, but sadly lacking that something to make it pop. Id decided to try to muscle through that, and was pretty much settling on Iron Warriors, and was half toying with the idea of Pre-Heresy... But only half. My bits box is deep, but not that deep and I cant justify all the extras Id want to get to myself right now.
Then a random thought hit me: do a What If army. What if Perturabo had sided with the Imperium during the Horus Heresy? If the story progressed as standard, just with different actors, that could only mean that Rogal Dorn had followed Horus into damnation.
You see what I just did there? Thats what I call a twofer. Two ideas for the price of one. So, Imperial Iron Warriors, and Chaotic Imperial Fists, both in the "standard" 40k universe. Im playing with the painter right now, Ill toss up some test ideas in a bit. :)