Something else to consider is that CoM and KH2 focus a great deal on the importance and power of memories; then there is the idea from BBS where they can follow the connections between their hearts to find each other. Taken together, it makes some sense. Whether they like it or not, the fairies and Maleficent are connected; if Maleficent was just in the Realm of Darkness, the fairies remembering her may have caused her to see that connection and follow it back to the Realm of Light.
Ursula...is much more difficult. It didn't seem like they actually killed her in KH1 the way they did in KH2, so maybe she also went to the Realm of Darkness when she vanished into the dark clouds. Or she became one of those creepy plants she was fond of turning people into and it took her a year to figure out how to change herself back. The more pressing concern is why Ariel fell for it a second time, but that would be getting off topic. xD
So Maleficent wasn't really dead. She was in the realm of darkness but needed help to return to the realm of light and used her connection to the fairies to do so.
Oogie Boogie wasn't really dead either. If i'm not mistaken Oogie's brain bug survived in KH1, and as long as his brain bug survives then Oogie Boogie is still alive. Maleficent was just able to return him to normal. Thankfully though Donald squishes his brain bug in KH2 I believe, so Oogie Boogie is really dead.
Ursula is the only one that doesn't have an explanation. But I do think your idea of her turning into one of her creepy little plant people would have been a good idea. I can easily imagine a scene where Maleficent finds Ursula in her creepy plan form and uses her magic to return Ursula to normal. That would have been a great idea.
So I'm wondering why nobody decided to revive Xehanort one of the times he's died, if they can revive them with the powers of darkness, why not Xehanort?
Wait, if she was in the Realm of Darkness, does this mean all dead people go to another Realm?
Xehanort never died though. Also it seems the power of darkness can't really revive people. Maleficent and Oogie Boogie weren't really dead. Ursula is the only one unexplained but i'm just going with they got lazy with Atlantica's story/
No, dead people don't go to the realm of darkness. The story hints that heart reincarnation is the rule in the series. It has been said that Kingdom Hearts is the place all hearts are born and all hearts return to. Some people interprete this to mean when someone dies their heart returns to Kingdom Hearts and then will be reborn into a new life.
But then we also have the soul to deal with. So far the soul simply seems to be the energy or life force that fuels the body. I use the example of a car. A car and all of its parts is the body, the gas that fuels it is the soul, and the driver which controls the car is the heart.
However Nobodies have a will of their own, yet they lack a heart. Perhaps though since heartless act on instict, the will of the heart remains with the nobody just as memories remain with the nobody. Perhaps this combination of memories and will causes nobodies to have what may be called a phantom heart, a mere shadow of the heart they once held. Only able to pretend to have emotions based on the emotions they felt in their memories.
Then we have Terra's Lingering Sentiment. Exactly what a lingering sentiment is and what makes up a lingering sentiment is unknown. But could the soul possibly have something to do with it? Perhaps a person can leave a piece of their soul behind that has been imprinted with some emotions and memories, which would explain how Terra's armor was able to take on a life of its own, it would need a soul for that, right? So could Lingering Sentiments be seen as a type of ghost?
Finally we have the underworld where we have those little ghostly orbs floating around. These could possibly be souls since in alot of Japanese animes Souls do look like ghostly orbs. But perhaps these ghostly orbs aren't sentient, but instead are more like residual energy. In ghost hunting communities residual energy is not the same as a ghost. A ghost is the sentient soul/essence/spirit of a deceased person who decides to remain in the world of the living for whatever reason. Residual energy on the other hand is spiritual energy of a person that is more like an imprint of the soul left behind, often like a recording that keeps playing over a person's death or a traumatic or emotional event that took place at a certain location. So could the little ghostly orbs in the underworld be that?
Regardless, all of this stuff is interesting to think about and considering that we have another saga after KH3, i'm sure one day we'll have the answers to all of this stuff.
