I have two educated theories about upcoming mario games:
Thank you Joe first off for making the Game List with videos
Super Paper Mario: Although this one is slightly less interesting and you may have already known that there is a quite advanced menu that has more RPG elements than just changing your character from Bowser to Peach and yes you can play as both of them. There are lightning attacks, fire attacks and ice attacks and also the Pixls who offer you super powers like bombs and grappling things at apparently no expense. So those of one who wanted a side-scrolling head bopping adventure you got it and those of you who wanted a turn-based RPG well you have half of it. Also, now this one is based on very little information, it seems that we might not be playing sideways after all an on screen arrow pad indicated to push up this could also mean right (As it would be sideways) but it is featured in many videos.
Super Mario Galaxy: Not much more to tell here but I did notice while watching the video a portal like object. All the videos we've seen seem to be the same setting about 10 different platforms, they seem to go out pretty far but only within eyeshot. So it is possible that there are multiple portals like in all of the past 3D mario's.