Saying that RG's story arc ended in KHII is too simple. You could also argue that KHII's ending had a definitive feeling to it.
In KHI Sora is seperated from his friends and he must find them, in the end Kairi is rescued but Sora still needs to find Riku. In CoM Riku struggles with his weakness of giving in to darkness and realises the best way he can help his friends is to accept the darkness in him. In KHII the three friends reunite and fight together (even if we barely see Kairi fighting on-screen, she does mention that she doesn't want to be left out anymore), and they return home, all is well.
Yes, I know in the post-credit scene they get a message from Mickey, but SoRiKai's expressions (neutra, happy and worried) were chosen so the message in theory could pertain to anything, not necessarily a new mission as became a must with later games.
How do we know that Radiant Garden is problemfree? The Xehanorts could easily stir up some trouble if they so desired, and we know some of them visit, so why would it be so improbable to find a conflict there?
And as others have mentioned, how do the Restoration Commitee react to there all of a sudden be a scientist walking around in Ansem's Lab, an area that they probably consider off-limits for people not part of the commitee. And they previosly had trouble with the Organization what is their reaction to having to work together with a former member of that organization?
The FF characters are not just like the Disney stuff where it's the same characters as the source material. FF presence was used by taking an already existing character and see what would happen to him/her if they were in a different environment.
During the prologue in KHI you are asked questions by three kids. If it wasn't for the fact that the journal stated they were FF characters I would have not known. By the time of KHII I was an avid FF player, but for many of us the FF characters, precisely because they are so different from their original counterparts, are just as much KH OC's as Mickey, Donald and Goofy.
Those three are Disney characters, but are being treated as main characters in the franchise. KH wants us to think of Mickey Mouse, the (supposed) symbol of innocence and goodwill, as a fierce master warrior. Okay, but I just have a hard time to feel an incentive to do so, when very little character development is given to him.
At the same time we have Squall who, due to feeling guilt over his inability to protect those he cared about, shed his name and took the name Leon. Here there was potential to give him a character arc that could end with him finally feeling he's redeemed himself and ready to take on his old name again, an arc that might have had something to do with those Rinoa wings from the KHII ending. Now KH has always been bad with character development, but with less and less FF pressence in this franchise, where they, I reiterate, function just as much as KH OC's as other characters do, it's just a massive missed opportunity.
It also feels wierd that this world, that has had so many important plotpoints throughout the franchise, does not get to finish its own story-arc.
BBS: The Capital of Light
Post-BBS: Fall from Grace
KHI: Dark empty husk of former self
KHII: Rebuilding
KHIII: What? You expected something like "Restored Glory"? Silly consumer of our product, tsk tsk tsk.
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