That almost works, but I think it would work better if the AL requirements, the name of the Dark incantation (and additional ones that are more fae / less demonic), and - for some heritages - the DR were all to change to reflect the different heritage.
A Warlock with demonic heritage - or that gains their powers from a demonic pact: it makes since that such would grants DR / cold iron. But I would expect a devilry heritage or pact to have DR / alchemical silver and an AL shift to any evil or any lawful. Similarly, while I would expect the DR for fae to remain the same (cold iron), I would expect something akin to any non-lawful for the AL as well as some renamings (dark incantation -> wild incantation), some new fae-ish style incantations (esp for higher levels, such as perhaps Otto's Irresistable dance for a Wild Incantation or something along those lines). For celestial heritage I imagine that there would be Light or Bright incantations, and the AL requirement would either by any non-evil or 'any good or any chaotic' (if eladrin descendent / pact) or 'any good or any lawful' (for archon descendent / pact).
The changes would be minor - for many of them merely a renaming of some features, a possible slight alteration to DR, and a possible alteration to AL requirements, but it would make the class more interesting, in my opinon.
For instance, I thought its take on celestials granting such pacts was decidely odd. It stated that chaotic good celestials might consider granting such powers because they are more fae than celestial. Then it stated that such warlocks would likely be chaotic good (the only good version allowable) but insisted that the power could eventually corrupt the mortal despite its source and said to see the PrC that 'redeems' the Warlock (Enlightened Soul). My question is, if the Warlock is so Good that a celestial would be willing to grant that mortal additional power to combat evil, then why would the mortal need redemption in the first place?
The developers at WotC seem intent on keeping the Warlock within a couple steps of Chaotic Evil, minus True Neutral. I just don't understand why.
I would suggest the following:
Devil Heritage / Pact:
Replace AL requirement "any evil or any chaotic" with "any evil or any lawful"
Replace levels 3, 7, 11, 15, 19 "DR / cold iron" with equivalent "DR / alchemical silver"
Fae Heritage / Pact:
Replace AL requirement "any evil or any chaotic" with "any chaotic or any neutral (excluding LN?)"
Replace levels 8, 13, 18 "Fiendish Resilience" with "Faeish Fast Healing" or "Fae Resilience"; same idea, but the former reflects the fact that Fae are not really all that resilient. Several have Fast Healing, however.
Replace level 16 "Dark Invocation" with "Wild Invocation", and perhaps keep the more fiendish style Dark Invocations as "Dark", thus preventing their use for one not of that heritage / pact. Create a few new "Wild Invocations" that match well with the concept of a faeish heritage or pact.
Celestial Heritage / Pact:
Replace AL requirement "any evil or any chaotic" with "any good or any neutral (excluding NE)" Or, if the celestial source is not NG, then replace with either "any good or any lawful" (for LG celestial, replacing DR / cold iron for DR / silver as well), or "any good or any chaotic" (for CG celestials).
Replace levels 8, 13, 18 "Fiendish Resilience" with "Celestial Resilience"
Replace level 16 "Dark Invocation" with "Light (or Bright) Invocation", and perhaps keep the more fiendish style Dark Invocations as "Dark", thus preventing their use for one not of that heritage / pact. Create a few new "Light (or Bright) Invocations" that match well with the concept of a celestial heritage or pact.
Draconic Heritage / Pact:
I'm told that Draconic Adept already has this covered, in theme and in style of crunch.
??? Heritage / Pact:
What other potential heritages / pacts might exist? A Lawful version, perhaps? An Undead one, maybe? Offhand, I cannot think of a realistic one for either.
Fae are not epitomes of chaos - that falls to the slaad, but I cannot see them granting such, let alone using something as lawful as 'pacts' - which are in effect a contractual agreement. I can't see modrons (the opposite of slaads) granting such powers either. Is there a prime equivalent of a lawful bent, the 'opposite' of fae? I cannot think of it.
Undead as a source is interesting, but I think that is covered in the spawning rules for undead conversion (such as vampire spawn created by a vampire, or new wights created by other wights, or ghouls created by ghasts, etc). Perhaps some powerful vampire or lich might create such, but I think the Necropolitan template / feat might best describe such a being, maybe.
If we did make an undead version, I imagine that it would be based off the vampire - which, I might add, many of the invocations of the Warlock already well mimic. So perhaps for such we would focus on the fiend aspect - and thus use the Warlock as is, perhaps altering the Fiendish Resilience to Undead Resilience and the DR / cold iron to DR / alchemical silver.
