and White only recently arrived in North America, but it sounds like Japan has another title in the works for their most popular franchise.
Dengei Nintendo DS teased a new Pokemon game in their most recent issue (via Siliconera), which suggests that we're due for another spinoff soon. It is, after all, a little early for Pokemon Grey.
Assuming this is another handheld title, it's not unreasonable to assume that the next title will be another Mystery Dungeon or Pokemon Ranger title. It's not uncommon for Dengeki Nintendo DS to cover Wii info as well though, so there's always a chance that Nintendo is planning another battle simulator.
Consider this a rumor for now, but expect more info when the next issue of Dengeki Nintendo DS hits stores on May 21st.