I can't remember where I read or heard it as it's been a while since I've looked up kh stuff, but I've heard that the secret ending just had ideas they could possibly use for a sequel but hadn't planned much out at that point. I'd say they probably wanted to do more and put the seeds for a sequel while also making it a satisfying piece on its own in case it wasn't successful. It didn't sound like most of the planning for kh2 began until after the first game was released and deemed successful. I'll have to see if I can find that info in more detail again.