Some folks at Bolter and Chainsword wrote up an alternate history version of the Heresy, with Horus staying loyal to Terra and Rogal Dorn falling to Chaos. There's a short version here and a longer PDF writeup here.

It's basically a straight swap for the Loyalist and Traitor legions from the canon universe, and while the Space Wolves falling to Khorne isn't hard to imagine, but really like the idea of the Blood Angels worshipping Nurgle.

So yeah, I'd be curious to see what others think of it.