Something I've been thinking about for about a week and that is 4-legged playable races in MMORPG games. I haven't really played any other MMO's out there other than WoW and maybe Skyforge (like spent an hour on that one) but I will look through other MMO games. I've not yet seen a 4-legged playable race.
But with WoW...
...WoW has the potential for it, in my opinion. Here are my thoughts:
#1. Both Keepers/Dryads and Centaurs share a lineage. They both are descended from Cenarius. While the Keepers/Dryads are directly descended from Cenarius, the Cenatur come from Zaetar who himself is a son of Cenarius. Since they then branch off into their respective race that leaves the door open (again in my opinion) to be brought into the major factions for conflict.
#2. Keepers/Dryads already fit the Alliance aesthetic since they more-or-less just look like Night Elves with 4 legs and Centaurs fit the Horde aesthetic looking more brutish and rugged with tan-like skin.
#3. Centaurs have had their contention with the Horde no doubt. We have seen this since WC3 and vanilla WoW. Development of a separate clan of Centaur that aren't barbaric and murderous isn't too far-fetched of an idea and we have seen races like Worgen who were developed into playable races apart from their original "feral" incarnations.
#4. I may be wrong but I believe the in-game models for Keepers, Dryads, and Centaur are all different. Personally I would love to seem them updated much like all of the races were in WoD so they don't look dopey and their animations are more fluid. Or, alternatively, you could develop a brand new model between male and female and just re-skin/re-texturize them between Keepers/Dryads and male/female Centaur. This leads me to point #5:
#5. Armor customization would be a sticking point. As we see with Mechagnomes, who get certain racial customization that overrides armor appearances, Blizzard seemingly does not view that as a major issue. The only thing I can really think of that would an issue with this Keepers/Dryads/Centaur idea would be for belt and pants appearances.
(Apologies for distorted pictures, best ones I could find that showcased the look as well as would show up in this post)