Classic Mode could use a bit of addressing, actually. It felt rather lackluster in Brawl - I mean, more lackluster than usual. The removal of character-specific Target Smashes, and any other type of bonus game in the middle, certainly didn't help that.
It also doesn't help that 1P Mode in the N64 game was pretty tightly designed around the smaller roster it had. Since there were only 8 characters, each one could have its own specific gimmick. DK was always giant for some reason. You would always fight a multitude of Yoshis and Kirbys. Mario would always have Luigi by his side, provided you'd unlocked him. Obviously, a larger roster from which characters are randomly selected for your fighting pleasure means that such intimate, personal touches like this aren't anywhere near as feasible, but still, I note the loss.
Maybe they could mix it up by reintroducing that aspect. A few characters would go back to having specific gimmicks surrounding them (fighting 30 of really weak foes, fighting a pair of related characters at once, fighting a giant opponent that actually kinda makes sense being giant, maybe some additional new ideas like fighting cloaked foes or something), and every time you reach a stage that calls for that gimmick, a character actually assigned to that gimmick is called up instead of arbitrarily applying that gimmick to any random character.
I'm not sure if it's a good idea or a bad idea, but it definitely is an idea.
Oh, and I recall chuckling a bit when Brawl's arbitrary Metal stage decided to give me Sonic. Metal Sonic. Only shame is that it wasn't the actual Metal Sonic (although that'd be an insanely kickass reference for Smash 4).
Brawl's Classic Mode actually did have the odd specific gimmick, or should I say each match up could be one of a few things.
Shamelessly stealing this from a Smash Wiki
Level 1 - The Legend of Zelda, Team or Normal
Level 2 - Yoshi or Donkey Kong, Giant (2 allies), Team, or Normal
Level 3 - Pokémon, Giant (1 ally), Team, or Norma
Level 4 - Fire Emblem or EarthBound, Giant (1 ally), Team, or Normal
Target Smash
Level 5 - Kirby, Giant (1 ally), Team, Multi, or Normal
Level 6 - Metroid or Pikmin, Giant (no allies), Multi, Metal, or Normal
Level 7 - Star Fox or F-Zero, Giant (no allies), Team, Multi, Metal, or Normal
Level 8 - Super Mario Bros, Giant (no allies), Team, Multi, Metal, or Normal
Level 9 - Game & Watch, Kid Icarus, Ice Climber or R.O.B, Giant (no allies), Multi, Metal, or Normal
Level 10 - WarioWare, Metal Gear or Sonic the Hedgehog, Giant (no allies), Multi, Metal or Normal
Target Smash (1 level higher, if played on Intense, level 5 again)
Level 11 - Free for All with three computers on Final Destination
Level 12 - Master Hand/Crazy Hand Battle
Personally I really hated this setup