Adventure mode should be changed to 'story mode', and each character should have their own kind of series of fights. Perhaps Master Hand has been warping the Nintendo universe, causing all kinds of characters to fight each other, just so he has the chance to fight the strongest?
Here's what I mean... let's take Mario for example.
Mario starts off in his character-specific area... the Mushroom Kingdom in this case (Link/Zelda/Ganondorf would start in Hyrule, Samus in Zebes, Kirby/Meta in Dreamland, etc, etc). He goes through a little platforming stage with enemies from that world (goombas/koopas). At the end, he finds a portal. This will lead to another random spot in the Nintendo universe - e.g. Hyrule, Zebes, Corneria, Peach's dressing room, etc.
There, Mario will fight his way through the stage, and at the end, fight a 'boss character' - a random character from that series. So if he goes to Hyrule, he might fight Link or Zelda or someone.
After fighting through 5-or-so of these stages, he'll face his 'rival'. Mario's would probably be Bowser (if the character has a lot of enemies, then it may have to be a random selection, e.g. Mario might get Wario instead of Bowser). Then he'd be warped to the Final Destination, revealing that it is Master Hand... and not Bowser behind this. After fighting Master Hand (and perhaps Crazy Hand), Mario does a victory dance, gets his Adventure/Story Mode trophy, we see a nice little montage of him beating up several characters, and then a nice little screenshot at the end.
I know, looking into it a bit... but y'know... XD