I think it might be Radient Garden, why? Well we know it's gonna be an orginal world since all kh games have orginal home bases (kh1: Traverse Town, COM: CO, Kh2: Radient Garden). So I don't think it'll be Sunset Herizon since it dosn't appear like there is life (meaning a barren wasteland) but again like Oathkeeper said if there's some sort of city then yeah I see that place highly likey.
The next place is Destiney Island I don't think it'll be that place either, why? Well because Sora and Riku are there and if stuff like heartless came there then wouldn't you think they'd remember that. Plus other than the village I don't see all the much they can do with the world. Though it puzzels me if Terra, Ven, and Aqua were once on Destiney Island they why dosn't Riku or Sora remeber them!?
What do I think is the most likley Radient Garden. Mainley because there is soooooo much left to that world a younger Leon, Cloud, Areith,Yuffie, and Cid could appear there since that was there homeland until they had to flea to Traverse Town. I would love to see how beautiful that area used to be (hence the name Radient Garden), we could explore areas like the Hollow Bastion castle, and perhaps find out the back story on the emptey castles which we see Malifcent, and Pete in kh2 (right after the Steamboat Willie world...I think) and why they were orginally there and perhaps explore them. Finally the whole story with Ansem the Wise, and his apprentices, and who wouldn't want to see a young Kiari? And perhaps find out who's her daddy...lol. But yeah if any world had to be a home base world I'd go with Radient Garden just because there is SOOO much story, and secrets left in that world!