Speaking of Templars...
Do you think Templars and Defilers should be playable if such a game is made?
Defilers, absolutely. There'd need to be some sort of consequences for it, of course, but I can't think of any particularly compelling reason not to include them.
Templars, probably not. Templars are too entrenched in the bureaucracy to really be adventurer-types, so if you were going to include them then it'd need to be a Templar-centric game, along the lines of Inquisitor or somesuch. Since Templars have every incentive to never leave their city of origin, they'd need an entire storyline full of intrigue just for them. Such a game would likely be lacking many of the elements that most people are looking for in a Dark Sun game, though. Templars also make such wonderful antagonists that it would be strange to include them as playable classes, also, since simply having one in your party would close off any number of good plot hooks available to the designers.