Templars definitely seem like a Paladin subclass to me in a 5E context. Probably the only Paladin subclass (though yeah maybe genie-based ones also work so long as the Templars are out to get them).
I don't think Dark Sun works as Dark Sun unless you do lock out some options though. Hopefully if they don't half-ass the hell out of this and just do "Dark Sun: Don't Offend The Billionaires Edition", but any kind of real Dark Sun (the defining features of that being "
Wake up gladiator, we've got a sorcerer-king to kill" and "powerful people ruined the world from greed and lust for power") should probably list the subclasses which make sense for the setting, before the immediate and inevitable disclaimer that obviously DMs can allow what they like in their actual game. Also they need to keep "No Clerics" as the
default (again actual tables can do what they want). It's actually easier with 5E as it's insanely easier to heal up.