^Oh but you misunderstood. The next series will be called Kingdom Hearts, well because that is what it is, at the end of the Secret Ending, it was labeled KINGDOM HEARTS, you can't change the name just because of a new installment. The next game just won't be a direct sequel and it won't be labeled Kingdom Hearts 3. Also its meant to tie up some key points of the Kingdom Hearts series, such as Xehanort's past and the Keyblade.
So basically the next game for the most part involves around the 3 Knights because it was hinted and the tall Knight who picks up the Kingdom Key, E.S (Enigmatic Solider), would most likely be the main character of this next game. So the game pretty much is set in the past but it links up things that will lead up to the future of the series. I consider it building up matter to be used for sequels. Also there will be more installments in this new series.
With that being said, Sora won't be directly involved with this game, but it still has something to due with Sora's story because of the Keyblade and such. But in my opinion Sora's story will pick up eventually after the main parts has been built back up, opening room for him returning as the main character in the future.