kingdom hearts 3As of writing (July 2014), Kingdom Hearts III is indeed planned and has been officially in development since E3 2013 (it's slated for 2015, but given this is Square, who knows when it will actually appear). Dream Drop Distance was the final bridge towards it, and it drops a lot of hints as to what it will contain. There may be scant information currently, being, there has only been an E3 trailer in June 2013 showing the updated graphics and Sora finding Eraqus' Keyblade, which was last seen in the possession of Aqua. At E3 2014, another trailer was teased, but this time simply of a younger Eraqus and Xehanort talking. However, through inference it is plausible to say that this is the final showdown against Xehanort.
It will also likely involve, in some shape or form - the Seven Lights against Xehanort's Thirteen Darknesses; rescuing Terra (I am firmly leaning towards that in the interim, Lea will probably stand in for this role as a Light); awakening Ven; and retrieving Aqua from the Realm of Darkness. Nomura has stated that Sora, Donald and Goofy will find the lost Guardians of Light whilst Riku and Mickey search for previous Keyblade wielders. Goals that will actually intertwine, as it were.
But what about Kairi?
Well, as per the ending of Dream Drop Distance, it is safe to say she is in training at the Mysterious Tower, but there's no word on what she will be doing. It is probable Lea will be her training partner whilst Riku is away and Yen Sid may - or may not - come out of retirement to help. But will she leave the Tower? Maybe. I'd like to think yes, as she - and Lea - would want some training experience. Personally, I can see Kairi and Lea going off to do a few missions together and helping to save worlds, and possibly going to Radiant Garden. Those who were once part of the Organization have not been touched upon, and I think out of the three heroes, Kairi is the one who would actually want to help them and would be able to help them. Sora defeated them, Riku despised them... but Kairi's heart is big, and she would forgive. It's a side plot, more than anything, but it's something I would love to see, especially as it is her original home. I'd also like her to be the one to find Aqua, for inheritance reasons, but we shall see how that pans out.
Actually, at the front of my wish list has to be Kairi and Lea talking and Kairi forgiving him, and then whilst out fighting Heartless, Lea just watches for a moment, stunned, as the Princess kicks the Heartless to the curb. I just really want her to get her moment to shine rather than been a token Guardian. Equally as much, I'd love for her to be the one to swoop in and save the boys, being able to handle herself and keep them safe. Hopefully, I'd say. Definitely, given her smirk at the end of Dream Drop Distance. If Xehanort isn't running, he should be.