Alright so, I think I've solved some of the main mysteries regarding the series around the world of union cross but I'm no expert so if I missed anything or you could add to the discussion that would be great, but I really think I'm on to something here
So, what we know so far, the world of Union Cross is a virtual replica of the world of Chi which is the actual story of the events that led to the keyblade war of old then after the world perished and was reborn the the data of what happened was used in creating a virtual copy which would be used to study that history and how to prevent that scenario from happening.
After the events of the war take place in Union Cross we start meeting new characters that weren't present in the original Chi, the first one we want to focus on is Ventus, for obvious reasons, we know he comes from a time that's definitely after the keyblade war, after the worlds were reborn and separated so how is he back? another charater we meet is Lauriam the original Marluxia(and just now we were introduced to Erlena who later becomes Larxene) what's important about Lauriam is that he looks noticeably younger than Marluxia (we know people may change a bit when they become nobodies but nothing as drastic as aging 10 years) so I believe Lauriam is the key to connecting Ventus to this era, we have just been looking at it backwards.
It is not that Ven traveled back in time, but the world of Union Cross was created much much later, specifically around the Birth By Sleep Era, there is a lot of evidence that leads me to believe this is the case like
This would place Lauriam in the correct age range (he was probably born around the same time as Isa & Lea, whose younger selves we met during BBS)
Ventus is shown in his BBS clothes except missing one thing, his should pauldron which he uses to summon his armor, but! there is one instance during Birth By Sleep where he was seen wearing no pauldron, during his time training with Master Xehanort to extract his darkness before his heart is fractured and falls in a coma
This takes place during a flashback and there is an unespecified amount of time that takes place during the gap between Ventus' heart being fractued and his arrival to the land of departure, it was shown that Xehanort was ready to give up on Ven waking up and took him to destiny islands to die
I propose that during that gap Ven was used to experiment and research the new virtual world that was developed, but who created that world? Xehanort in an attempt to unlock the secrets of the past? No, he has never been shown to be know about creating that sort of thing, but we know of another character who has done just that, recreate worlds in the datascape and transport people there without them even knowing it
I believe Ansem the Wise created the world of Union Cross, true he has never been shown talking about the keyblade war but I think he is more connected to the past than we know, first of all, he is ansem the WISE, king of radiant garden, so it would make sense for a wise king to know about the events of the past that regular people ignore, even Kairi's grandma knew a bit about how the world ended and was reborn so it is confirmed that knowledge of the old world reached radiant garden at least in some form
So, Ansem the Wise managed to get a hold of Ven's body for a while, scanned him and added him to the world, maybe he thought that as a keyblade weilder his heart would help his simulated world be more accurate
Here's where I must say I DO NOT CLAIM TO KNOW HOW ANSEM THE WISE GOT A HOLD OF VENTUS, maybe Xehanort, having his own desire to know about the past allowed the experiment to take place and after he Ventus could no longer be useful, that's when he took him to destiny islands (Xehanort would obviously have to hide his identity since when Ansem find the amnesiac Terra-nort and hears his name he does not react to it, and it would make sense, Xehanort wouldn't like others, like Eraqus, discover what he was doing)
Earlier I said Ansem the Wise (ATW) may know more about the world before the keyblade war than he lets on and that there is something else that connects him to the past, something that I noticed long ago but couldn't find a solid reason for it to exist in such detail
During Kingdom Hearts we not only learn about ATW's knowledge of creating virtual worlds and encoding real world objects into data, we learn that the old mansion in twilight town serves as his hideout/research lab but if we explore the place and pay attention to it's surroundings we can find something peculiar, all around the mansions we see iconography of something very important, we see images and sculptures of unicorns adorning the whole placethe unicorn being the symbol of one of the foretellers in chi/union cross
So what if ATW in his research over the years found this information and tried to learn more about this old world (maybe he believed Ira as the Leader would be the one who held the answers so he aligned himself with his union)
Skipping over a bit, during the new cutscene added to Re:Coded we see Maleficent talk about the similarities to between the datascape and the tome of prophecies and a few weeks ago we discovered that the Maleficent in UC is not a virtual character but the true maleficent, whose heart traveled back in time after being defeated in KH1 and is trying to change her fate, but instead of traveling to her true past she was (somehow) trapped in a virtual past by a hooded character refering to himself only as "darkness", since they don't want her interfering with the future and the plans they have, this had led many to believe this "darkness" to be Xehanort since he knows about time travel and has big plans in store, but I don't think that's the case, I believe, again this darkness is our man, Ansem the Wise, he also has been conducting research all this time and would want an uncontrolled outside force to interfere with it, and taking care of the easy one, the figure calls himself "Darkness", what other name has Ansem given himself? DiZ: DARKNESS in Zero
So Ansem as the administrator of this virtual world would have no problem in controlling anyone inside his world/software this would help his research to continue on.
And I believe this research, this knowledge of the past studied to prevent a future disaster is the very same research that has been alluded by him all these games, the very same research that he hid inside Sora while he was asleep, the knowledge to finding yourself or your lost friends in your hour of need
Back to Maleficent, we know she traveled back in time and I stated how the world of UC was created around the time of BBS, but we don't really know how long it lasted so it could be that, maleficent traveled back to the time of BBS trying to undo her defeat during that game, but was trapped in a virtual world by Ansem and during the time she spent there the other events of BBS took place and then the betrayl by Xehanort and the other apprentices, when ATW was betrayed and sent to the realm of darkness that's when Maleficent, without Ansem eye on her, was able to escape and return to the real world, to the time of KH2
So in conclusion
Ansem the wise wanted to know more about the keyblade war in order to prevent it and protect all worlds, using the knowledge he gathered over the years he created a virtual simulation of that era (probably thanks to Xehanort's own plans) added Ventus in an effort to enhance the world by adding a true keyblade weilder, Lauriam and Erlena were also added (we could think of them as a sort of Beta testers), they might've lost their hearts at the end of the simulation in whatever way that happens, at the same time Maleficent is trapped in the virtual world by Ansem to prevent her from changing history, the simulation continues, Xehanort takes Ventus back to destiny islands and the events of the games take place from BBS to KH2 by this time ATW has been sent to the realm of darkness and returned as DiZ, maleficent escapes the datascape (because UC is just another datascape, that's why the world of the journal in Re:Coded seems familiary to her) and Ansem after experiencing all he has, and gaining all that knowledge over the years, takes his data and places it inside a sleeping Sora, hoping that somehow the experiences of the past help save the future
PHEW.... so that's it. I know some parts are out of order but im writting all this from ideas I had on my head and giving it structure as I go on, I hope you found in interesting and bring forth your own ideas about what you think I got right or not so right
Thans for reading!
May your heart be your guiding key