There will be one, Nintendo isn't about to let Mystery Dungeo die. It has arguably one of the biggest fanbases of any of the Pokemon spinoffs, and brings in a good bit of publicity and revenue. Two things that makes a game series good.
Yes, the next Mystery Dungeon game will be here, most likely after the hype for Black 2 and White 2 dies down. It's probably already in development, or at the very least planning stage.
This stuff takes time, I mean they have to come up with a plot (one with meaning, a few major twists, and an emotional ending) which is a feat in itself, then comes the actual game and software development and coding (not to mention all the work that goes into creating the dungeons themselves, I mean no two dungeon maps are exactly alike, that takes effort), and than come the goodies like the WiFi connections among other things they have to add to the game.
So yeah, that's my bit of rambling. The sum of that big wall of text is: they are probably working on it, it'll take some time.
There will be another one, mark my words.
Knightfall signing off...;005;