Interestingly enough, it was asked in
an interview with Nintendo and TOSE in 2009 whether there were any plans for a sixth game in the series where Hitoshi Yamagami of Nintendo replied "yes!" with a long pause and a laugh. This definitely doesn't imply that a sequel is coming, but suggests there probably might be one in the future. Nintendo also said this, so I suppose you could argue that TOSE, the developers would be the best people to ask.
I'd suspect it would probably be for Nintendo 3DS, I would love to play a new Starfy game. Curiously when
TOSE were asked a sequel was going to be released on Wii, they replied that they would "most likely have to make it into a 3D game which would be a natural extension of the series and felt that such a thing would make things 'too complicated for the general kid-friendly audience". I'd also prefer a 3DS version for convenience, but I don't think a Wii/WiiU version would necessarily make it a bad idea because they could probably make it a '2.5D' style game like Kirby's Epic Yarn rather than set the game in a full 3D environment.