What if...by theory. Sora and the rest of the gang aren't traveling to different worlds but traveling through time. And each world is a different timeline in the same world. This would explain how the heartless were able to spread so quickly and seemingly be there before Sora enters the world. How is it that the Heartless were able to beat him every time? Maybe they've always existed and what we fight is actually the evolved heartless. Like the theory of Evolution. This would also explain why Sora has to be a certain level to be in a certain world. (Like in the Gummi Ship in KH2 it would recommend you to be at a certain level)

 

Another thing is the FF7 crew. The events in FF Advent Children took place 2 years after FF7 (which the designs are based on in KH2). But in KH1 they were in their original designs. And the time between KH1 &KH2 was just one year.

 

We've also been discovering how time travel works in KH3D.

 

Just a though .3. also I got bored.

well heartless have been shown to just pop up in random portals of darkness. and we know portals of darkness have been used to travel to different worlds. so it makes sense to assume the heartless are doing the same thing. plus i think in the games the reason the heartless always beat Sora to a new world is because they were just already there. Sora and co. always arrive late to the party. 

 

as for the FFVII designs, i wouldn't put too much thought into it. i think the design choices were based on fan feedback and what would be popular. 

Well, remeber that the Heartless had a 9 year head start on Sora. 

Well human, Heartless are the manifestation of darkness within a being's heart, and with that being said, it'd make sense for them to already appear in a world before Sora and his goonies arrive, am I right? They come from the Realm Of Darkness, so finding a gateway to the Realm Of Light must have been easy for them, since then they'd just have to locate a source of energy to feed off of, such as the hearts of worlds! Hence, why they always manage to pop up early! I don't think time travel has anything to do with it, more like normal travel!

 

But then again, what do I know?