I wouldn't say Mario is dying, all his recent games have been fun. The 3D Mario games sort of took precedence over the 2D games since Mario 64 came out.
I have enjoyed every single New Super Mario game, but Nintendo doesn't realize we want something New (and not just in the title).
The challenge mode in Mario U is genius. Do it again, but more, three times as much!
Give us new music, new powerups that have more than one use, like the racoon suit had flight and tail swiping! That penguin suit was genius! It had sliding, ice powers, and it made waters levels easier to control! Do more of that!
Give us new villains like the RPG's get! Fawful and Smithy are worshiped by fans, do not relegate them to RPGs! My request is to bring Wart from Mario 2 into the real world (not Mario's dream) and make Wart battle Bowser for power, and have Mario side with Bowser's minions!
The 3D Mario games have been more creative, Galaxy 1 and 2 hold 10 times as much creativity as 4 New Super Mario games. Give each level something different, like a ball rolling level, a vine level, a barrel level, an occasional remake level from old games, just don't play on what made past games great!