A few things to point out that I’ve done before:
-Dark Iron Dwarves use to be all angry enemy mooks who were obsessed with doing nefarious deeds and blowing themselves up. They’re now more fleshed out and a playable race.
-Zandalari were once bad guys allied with the Mogu, one of the most evil races in the game. They got more fleshed out, became awesome with neat personalities, and became playable.
-Many of the Mag’har Orcs were either part of the Iron Horde or descendants of them. They’re part of our Horde now and playable.
-Gilgoblins were once just enemy mooks with low intelligence and obsessed with treasure. They were fleshed out to be something much more with the Kelfin and are a primate candidate as a playable race.
-Worgen were just angry werewolf mooks. They were implemented onto Gilneans and were something that wasn’t angry destruction all the time and are playable.
-Then of course you have undead of all sorts who use to be part of the Scourge that broke free and have been able to keep control of themselves. This includes Death Knights, but also includes things like geists, banshees, liches, and several other types of undead.
The point is that if Blizzard wants something playable, even if it doesn’t make sense with how a race currently is, things can always change with how a NPC race is and they’ll do whatever is needed to make them fit as a playable race, and San’layn shouldn’t be an exception. They’ve even made some enemy NPC races a lot more friendly in general, like the Rajani Mogu, so nothing is impossible to me in that regard.
I see no reason why San’layn would be an exception, given a lot can be done for them to make it make sense and work very well as a playable race. They have a lot of potential as a playable race, and it’s something I hope we see become a reality someday. 
Now my arm is broken though.