Neonchameleon
Legend
If you're ruling out 4e (and 4e Dark Sun is actually a much better representation of Dark Sun than 2e ever was - compare the Defiling rules for example*), I'd go right to the other end of the scale for my slightly heroic gritty setting; WHFRP 2e (or 1e if you have it but use the 2e magic system). Violent, gritty, survival based. Dangerous magic that can blow back in your face (spell blowback needs tweaking for defiling rather than accidently summoning demons). And honestly, for Dark Sun how can you say no to a system where your starting PC class can be gladiator or ratcatcher?
And if you want loose and open with wide DM leeway, and little mechanical optimisation you don't want P6. Or P-anything. I would point out 4e Essentials for simpler PCs tho.
* 2e: Defilers and Preservers are separate classes with the core difference being that levelling defilers costs less XP. Once you've made the decision to defile or preserve, that is it. You are permanently either a perserver or defiler whatever the fluff and tie-in novels say. 4e: Defiling can be done by any arcane caster and is a temptation mechanic; you are most likely to want to defile when your back is against the wall. And if you're routinely tempted you can make it far more effective. (WHFRP: Defiling would be a part of magic - although stick to lesser magics and lower power and you defile much less; "Preservers" would need to get the dice from Hedge Wizard and couldn't preserve on powerful spells anyway).
And if you want loose and open with wide DM leeway, and little mechanical optimisation you don't want P6. Or P-anything. I would point out 4e Essentials for simpler PCs tho.
* 2e: Defilers and Preservers are separate classes with the core difference being that levelling defilers costs less XP. Once you've made the decision to defile or preserve, that is it. You are permanently either a perserver or defiler whatever the fluff and tie-in novels say. 4e: Defiling can be done by any arcane caster and is a temptation mechanic; you are most likely to want to defile when your back is against the wall. And if you're routinely tempted you can make it far more effective. (WHFRP: Defiling would be a part of magic - although stick to lesser magics and lower power and you defile much less; "Preservers" would need to get the dice from Hedge Wizard and couldn't preserve on powerful spells anyway).