When are you going to post your Adventure Mode concept, Omega? I'm still waiting.
Anyways, for Adventure Mode, how about we have secret levels in some of the stages? The secret levels are basically platforming challenges and obstacle courses - reach the finish and you get a reward of some sort before returning back to the normal level at a different, farther point.
The secret levels could even be Board the Platform levels; after you complete one of these hidden Board the Platform courses, you unlock them as a Stadium mode for all characters.
Also, to relate this idea to Hex's story and whatever Omega's ideas are, the levels could be a conglomeration of either previous stages the player has completed, or even elements of the franchise of the chosen character. For example, for a Mario secret level, it could be designed like
one of the secret courses from Super Mario Sunshine (starts at 1:25). For a Lip secret level, it could involve her jumping across a long stretch of falling Panel de Pon blocks to reach the finish.
In addition, since Master Hand and/or Crazy Hand are designing these courses, for a unique graphical trick, the courses will build themselves as the player moves through them. As the player runs across one platform, a new platform fades in and so on. This shows a piece-by-piece thought process of Master Hand and Crazy Hand.
Heck, even the Crazy Hand alternative scenarios idea I suggested earlier could work here. The player is almost done with a level when Crazy Hand decides to lengthen it by double it's length all of the sudden.