I've watched every single Xmen cartoon, I've even rewatched the old one from the early 90s, which doesn't hold up well at all. I couldn't make it more than like 5-6 episodes through Evolution, it was just unbearably terrible. But it does set a precedent for both Magneto and Storm as Horsemen. Not that they really need a precedent. Polaris is probably just as powerful as her father and she was made into Pestilence at the same time Gambit was turned into Death.
Besides, it's not like they can make an Xmen movie without Fassbender. The dude has been absolutely brilliant, and is a big part of why the reboot has done so well. I figured he'd be working with Apocalypse, just didn't know they'd go so far as to make him a Horseman.