Pokmon Mystery Dungeon: Explorers of the Sky is a continuation of the previous games; Explorers of Time & Explorers of Darkness. Featuring the same Dungeon crawling gameplay, Explorers of the Sky expands upon the story showing what happened to Grovyle the Thief after the storyling and also shows how the Wigglytuff Guild comes together and even has you as playing as some of the founding members of it outside of your Pokmon and your Partner.
The full extent of the story is not yet known nor is it known how many areas or characters from the previous games will return. This game removes 2 of the starter Pokmon from previous games and adds 5 more; Vulpix, Eevee, Phanpy, Shinx & Riolu. It is also expected to feature Shaymin and its forms as well as the new forms of Giratina & Rotom that were introduced since the last game.
This game is expected to be released in Spring of 2009 in Japan and we'll have more info added from now to then.