Sabor the leopard is never coming back.
The moment Sabor died was the moment Sora became a hard-boiled mofo.
While that may hold true (as any other element involving Deep Jungle), the possibility would still be there even for that Leopard. ;P
I know this, I never said otherwise.
Ansem is a bit of different case dont you think? Maleficent backs your point but Ansem wasn't truly dead, he (or a part of him, either way) remained in riku after his defeat in kh1.
Again Ansem is different to Maleficents case. Maleficent backs your claim more than Ansem.
Not saying your wrong mind you just that Ansem's situation wasn't the same as Maleficent, a part of him was there to return whereas Maleficent only had her cloak and raven left behind.
There may be variances in their dispositions, but the basic situation is the same. For all intents and purposes, both Ansem and Maleficent where existing no more on the physical layer of reality. According to Vexen, Maleficent was in the Realm of Darkness while Ansem was inside Riku's heart, that's merely a difference in location.
Maleficent wasn't "truly" dead either if we go by Vexen's explanation, as he only said that she's "gone" and cannot return of her own volition, but never actually ruled out a return in itself.
All in all, I was also pointing out more the process itself rather than looking in detail on Maleficent's or Ansem's individual cases.
Like I said above, there are some variances/differences between them, but the core process as to
how they came eventually back is the same.
It's clearly not that simple as if it was everyone that had "died" would return simply because they were remembered by their loved ones (or enemies).
And we both know a memory doesn't die with the person the memory is based on since Sora clearly remembered Maleficent as did Pete. (though them remembering didn't bring her back)
Your part on sleep is the key factor, Maleficent once said she has "power of sleep" and as we know even if the body croaks the heart is always left behind.
What may determine resurrection may more rely on waking the heart that has fallen into slumber, thus also adding substance to the line "birth by sleep".
You're completely correct on that one, it is obvious that we are still missing one or more pieces of the whole process, namely exactly the piece(s) that prevent this process from happening naturally and every time.
It is an oversimplication, yes, but a starting point from which we can go further.
Sleep is indeed another key factor, as we can see in DDD when looking at the sleeping worlds (worlds have hearts too and therefore work exactly like persons). Some (the majority actually) of the worlds restored at the end of KH 1 had their "Birth by Sleep" and were restored, but several of them didn't and they remained in sleep,
eternal sleep even until Sora (Riku actually didn't do anything because he was in Sora's dreams) manually opened the Keyholes of Sleep.
Sora himself also awoke only after 1. Riku defeated the Nightmare trapping his heart, 2. opened a Keyhole on Sora's heart station (most likely Sora's personal "Keyhole of Sleep") and 3. answered the questions of Roxas, Ven and Xion which was, in Ansem the Wise's words "the last key" to freeing (and awakening) Sora.
Dream Drop Distance said:
Ansem the Wise: He's not here. You see, by defeating the Nightmare
imprisoning Sora, you freed him.
Riku: You mean...the Sora wrapped in that black phantom?
Ansem the Wise: Then, you came here, and you were questioned by three
young people. That was the final key to awaken him. Sora is awake. You
can go home now.
So, as I theorized before DDD came out and as you point out in the last sentence, there are several conditions to fulfill in order to truly "revive" someone, explicitly waking the sleeping heart itself being another one of them, not to mention we may not know every condition yet.
Not to mention that for existences who consists only of a heart, like Naminé, Xion, Roxas (and possibly Eraqus) right now, the process might be even more complicated.
This explains also why it doesn't happen all the time or in general more often. We only saw/see it so often because there are several characters among the wider central cast (among them Maleficent, Xehanort, Riku and possibly Sora) who may possess all the needed abilities in order to fulfill all those conditions.
It is certainly not as easy (and cheap as some may say) like in Dragon Ball or Sailor Moon to bring someone back.