No, I'm not talking about Bowser's Inside Story, I mean I think there should be a spin-off game where Bowser is the main playable character.
There could even be fun and/or interesting gameplay mechanics implemented around Bowser. :)
The story would most likely revolve around Bowser finding himself upstaged by a new villain in the Mushroom Kingdom, so he decides to take down the new villain in order to retain his status as the kingdom's greatest threat.
So what do you think a spin-off with Bowser as the main character would be like?