If we go by logic, Nintendo normally announces officially a game releasing within the next year or throughout the Holidays. So let's go with this:
Super Mario 64: 1996
Super Mario Sunshine: 2002
Super Mario Galaxy: 2007
Super Mario Galaxy 2: 2010
Super Mario U: Probably 2014 or 2015
So no announcement is legit.
Mario Kart DS: 2005
Mario Kart Wii: 2008
Mario Kart 7: 2011
Mario Kart U: 2014?
Again, no announcement is legit. What do you want them to show, linecodes?
Wind Waker: 2002
Twilight Princess: 2006
Skyward Sword: 2011
The Legend of Zelda Wii U: 2014?
Seems legit. Just look at the cycles...