Kamikaze Midget,
as you remembered

preservers don't drain other life sources.
For my own self I never even liked this idea of "
oh they only take just enough..." so, that would still drain life..they still couldn't cast in deserts...so it makes *no sense* that Preservers drain life force at all!
No, much better to say:
Defilers are dirty sneaking cheats, lol, who came upon a method to suck the life energy out of other things to boost thier magic, where as ordinary arcane magic works just as normal (Preservers).
This makes it ALL much simpler: you are either a Preserver (and your magic works as normal regardless of arcane class) or you're a Defiler (in which case defiling rules come into play).
As for feats, I thingkyou didn't get what I and perhaps others meant

Defilers *ARE* more powerful than Preservers, harsh fact of Athas. They "cheat", boosting their spells at the expense of others.
So,
any arcane class, should be able to take "Defiling feats" and thus become a "defiler".
If you make defiling an "on/off" choice an effect they do TO their spells at will, this also makes the ability for PC or NPC defilers to hide amongst the population, easier
by this I mean, if a defiler caused defiling EVERY time he cast a spell or ritual...he would turn his immediate area to ash and be discovered damn near instantly, wouldn't he?
But if defiling is a "dirty trade secret", an "add on" to the
normal routine of casting, which you chose to add strength to the effect by defiling life energy form around you...then everything works out neatly!
You wouldn't know someone is a defiler until they use defiling abilities, hence even more reaosn why the average folk kill ALL arcane casters on sight, as you couldn't tell until a defiler was such, decided to abuse his power..and when he does..oh boy!!
Say, Joe the Defiler

is with an adventuring party, he casts his spells as normal, but during a climatic battle he uses his defiling powers to boost his spells, to help defeat a deadly enemy...well..his secret is out! But, his power is indeed, deadly.
You can imagine a base defiling feat "Defiler Apotheosis" or whatever you wish to call it, from that point on, there's NO going back to a Preserver, without a special ritual and atonement performed by a druid.
The effect is: you gain the ability to add +1 attack, +1 damage bonus with arcane attacks, at your choice, but in so doing, defile a close burst of 1 square per tier of caster, this does Int or Cha vs Fortitude, infliciting a -1 hit atack penalty until end of the defiler's next turn. killing simple plants etc automatically
as noted in 2nd ed, defiling causes a lot of pain, it makes it truly scary and nasty when it even hurts allies.
Other feats and associated powers are then allowable with levels etc.
"Defiler Draw" lvl 7 requirement, daily, minor action, you regain the use of 1 encounter power of your choice, but trigger a close burst 3, Int or Cha vs Fortitude, does 1hp damage per tier of caster.
"Defiler Ravage", lvl 15 requirement, daily, minor action, you regain the use of 1 daily or encounter power of your choice, but trigger a close burst 6, Int or Cha vs Fortitude, does 5hp damage per tier of caster.
"Defiler Overcharge", lvl 21 requirment, daily, minor action, you empower an arcane attack power to do more damage, it does an extra 15 hp damage to all victims.
but trigger a close burst 9, Int or Cha vs Fortitude, does 15hp damage.
Another feat would let the defiler spare allies from the defiling power. etc etc
Oh, templars as warlocks make perfect sense, by the way! they have a pact with their Sorceror king...
And as said if you make "Mutation" a schtick of Dark Sun, letting other races/classes exist is fine.