I honestly don't. It's usually quite a gap between Mario games. (1996 = 64, 2002 = Sunshine, 2007 = Galaxy) 

And there's usually only one per generation.

Honestly, I'd prefer if they wait a few years. I want them to put their all into it, just like Galaxy. It should have tons of new innovative ideas as well.

Yet, I still see people talking about "new Mario, new Zelda" for Wii. Why are people expecting another 3D Mario?

I think I remember Miyamoto saying something about a sequel to Galaxy. I think that's why people are assuming it's 3D.

I'd like a Luigis mansion using the wiimote

Another one for Wii is already in development. It's still 2-3 years away but it's coming.

http://kotaku.com/5029215/miyamoto-confirms-new-mario-zelda-pikmin-projects

Said Miyamoto:

"Making these 'traditional' games is what I am best at. Because games of that nature take upwards of two or three years to make, we always have to keep the teams working on those projects going. At any given time, the team could be five to ten people, or it could be 50-plus. People are always switching in and out of those teams.

"They are all working on more Mario, Zelda and Pikmin projects," he said. "And they all work in close proximity to me, so I can keep a good eye on them."

Me too

next Christmas maybe, but wont be a big new Mario, might be seen as more of a 'Lost Levels' to Galaxy.

It will come in a few years, hell I wouldn't be surprised if it came the same year S-E gonna release FF13, hmm that's year 2010 oe something?

First of all: what does Miyamoto understand with "traditional games"...

I think yes, there will be another 3D mario, at least, i hope so.

I think it will come because I expect the Wii to last longer than the Cube and the N64, and therefore, it will just reach for a new one to be released at the end of the gen.