dark realm
Concerning the structure of worlds, first Sora and the Disney characters, and where ever we dwell, these so called normal worlds are attached to the Realm of Light. If all of these exist at the same level and you imagine them all sitting on top of a plane, there is also a plane of existence on the other side where the Realm of Darkness exists.

At present, there are 4 worlds in-between these planes that have appeared. A) Castle Oblivion, B)Twilight Town, C) Yen Sid's Tower, and D) The World That Never Was.
So we've seen the in-between worlds and the realm of light. Are we going to be seeing the realm of darkness sometime soon?

what's next?
"The period of the King's absence", "The period of Riku's absence", "Roxas' time in Organization XIII" and "Xehanort's past".
Roxas and XIII (358). Xehanort (Birth By Sleep). So then there are TWO untold stories, although technically we can cobble together information for Riku based on previous games. Are we going to be able to play as King Mickey and will he be able to actually defeat things while in the Realm of Darkness?

more questions
The keyblade constructed from the Seven Princesses' hearts that Riku obtained in the previous game allowed for the keyhole in Hollow Bastion to appear. For the plan to cause darkness to overflow from hearts, the special hearts held be the Princesses were used to artificially make this keyblade. So it is different from a proper keyblade. There are further suggestions that this keyblade is artificial as well.
I don't remember if it was properly explained how a Keyblade could form from the Princesses' Hearts.

insufficient
There isn't necessarily one keyblade for the dark side and one for the light side, just as many as there exist people with qualified hearts. Concerning the conditions to wield one, at this stage, "Those with strong hearts" is the only obvious one. However, less obvious conditions still exist and there are still plans for an opportunity to reveal those.
So does that mean the Final Fantasy characters don't have strong hearts? Cloud was touched deeply enough by the darkness to wield its power but never became a Heartless. Leon tried to wield the Keyblade but it rejected him and he followed by saying there is only one wielder. Do you have to be utterly broken down to either hopelessness or empty rage to become a Heartless? What's the real condition for wielding a Keyblade?

Nobodies
When heartless are defeated, essentially the owner's hearts are rejoined with their once extinguished body.

In the case of a Nobody being defeated, it's a little more complicated. If the above mentioned hearts are liberated, they return to their original form. However, if the heart is still stolen by the heartless, the Nobody's body is swallowed by darkness. If somewhere in the world their hearts are taken back, perhaps they might be able to return to their original human form.
So the Organization XIII members are still floating around somewhere as Heartless? We've only managed to restore Xemnas/Ansem to human form by defeating both his Heartless and his Nobody. Where are the other eleven Heartless? Or perhaps it's simpler than that. Maybe Ansem wasn't solely the embodiment of Xemnas's Heartless (although not while considering information from BBS). Maybe the entire Organization compiled together into one Heartless while becoming individual Nobodies. Although it's not as interesting as the possibility of being able to identify the Heartless of the other eleven Nobodies.

Anti-Form
Concerning Sora's Anti Form, you could think that the reason is related to his changing into a heartless once before. In fact, beyond Sora's Nobody, you it is natural to assume that Sora is influenced by the heartless. Riku's Dark Mode is similar. By using such a strong power too much, he may have ended up approaching too close to darkness.
Why does Riku's Dark Mode make him more powerful while Sora is basically useless in his Anti-Form?

Roxas
when Sora himself was changed into a heartless he was purified by Kairi. In order for Sora to be revived without following the essential course, the special way Roxas and Namine were born from these types, special Nobodies ended up being left behind.
I realize that Kairi would not have a real Nobody of her own, but I am personally forced to wonder what Sora's original Nobody would look and behave like? Nobodies "use memories" to determine their personality and mannerisms without expressing any real heart or emotion so I guess Sora would basically be the same. But Sora was turned into a weak Shadow so he probably wouldn't have had a Nobody of his own anyway.

DI
Destiny Island itself ended up disappearing and for the people of that world time is stopped. Afterwards, Sora slept for 1 year, and then everyone's memories of Sora himself were no longer there. As such the memory actually became "On the night of the storm, Riku's body disappeared". Once Sora awoke, the memories of him returned.
Namine is the obvious source of this memory tampering. Obviously, it would kind of offset the flow if everyone was stuck wondering what happened to Sora. Obviously, nobody cares about Riku.

motivation
Ansem (Xehanort) seems to have thoroughly gotten rid of his own heart, but Xemnas (Xehanort) commanding Organization XIII is seeking a fresh heart.
Naturally, every living being demands what it doesn't have. Ansem not having a whole existence in lacking his body sought a whole existence by obtaining the power of Kingdom Hearts. Xemnas meanwhile lacked a heart and came to understand through cold logic that one cannot be whole without one. Unfortunately, his logic led him to believe he needed a stronger heart which he tried to obtain from the Kingdom of the Hearts of Men.