It looks like more details are coming in for Kingdom Hearts III, and things are actually looking pretty good. Although it can be expected, through a Famitsu Weekly interview it is now confirmed that the game will continue after the cliffhanger in Kingdom Hearts 3D, and will feature the "final battle [with] Master Xehanort," which the core entries have been leading up to this entire time. In short, although it doesn't preclude future Kingdom Hearts games, Kingdom Hearts III is not going to be a gaiden. and will finally bring an end to the saga that started all the way back in 2002.
Other bits of the interview include hinting at a playable Riku and Kairi, updated visuals to keep the game more in-line with Disney illustrations, an improved AI system, and a more badass Sora (now that he's gone through the Mark of Mastery Exam). More info will be revealed this October at TGS and at the D23 Expo in Japan.
If you need a recap of the franchise's story thus far, or just want to jump right into Kingdom Hearts III when it comes out, check out the "Kingdom Hearts story made easy" portion of my KH retrospective, at the bottom. It's actually not that convoluted when you take out all the side stories and "hearts darkness darkness hearts" dialog.
Kingdom Hearts III [KH Insider]