Three games and that's it. If it turns into a final fantasy type series all the games from that point on would suck. Seeing some kid who get's keyblades and fights heartless will get really boring and repetitive.
If that did happen you would probably get something like: In Kingdom Hearts VIII, You play as Umi (japanese for ocean) and fight the emotionless with the mysterious keyblade. Donald and Goofy are back.... again, and they join Umi on his quest to find his friends. Along the way Umi and co. will meet disney characters and Final Fantasy characters which have alreay been seen in the past 3 games. Such favorites being: Auron... again, Squall... again, Lulu, Cloud... again, Rinoa, and many more. Plus a mysterious group called the Organization Version 7 has plans in store for Umi but what could they be? Find out in Kingdom Hearts VIII, coming December 16th, 2020.
Not only that you would probably have over 100 worlds that you can go to, 90% perecnt of them probably being old worlds from the previous games with nothing new, interesting, or remotely exciting. Anyway my p oint being, it would not be a good idea.