OK, this is my understanding of the next installments in the KH series.

Kingdom Hearts: Birth By Sleep (PlayStation Portable) is a sequel to the entire series, taking place about ten years prior to Kingdom Hearts. Ven, Terra, and Aqua are some of the apparently many Keyblade Wielders. The worlds that you will visit are apparently the same, but the characters in those worlds are aged to match the time period, such as a younger Hercules in Olympus Coliseum, and possibly meeting Mufasa (Simba's Father) in the Pride Lands. There have also been hints at a possible meeting with Sora, Riku, and Kairi. (Perhaps it's this meeting that inspires Sora's nobody to take the appearance of Ven?)

Kingdom Hearts: 358/2 Days (Nintendo DS) is the story of what Roxas was up to during the one year that Sora was asleep. There will also be a fourteenth member of Organization XIII

(accidentally posted before I was done, sorry...) in the game, that is apparently female. King Mickey is also said to appear, although what his role will be, I don't know.

Kingdom Hearts: Coded (Mobile Phone) is the only one of the three games that will take place after KH2. King Mickey apparently digitizes the contents of Jiminy's journal, making a virtual Sora appear on Destiny Islands. It is also the least action packed of the games, being mostly about puzzle solving.

There has not yet been a Kingdom Hearts 3 title announced, although it will most likely be made for the PlayStation 3 sometime in the 2010s since Birth By Sleep will most likely be released in 2009.