Graphics
~Make the menus a bit darker.
~Free up the camera! Naughty shots are going to happen no matter how much you try to avoid it, and it's not going to change the game's rating!!!
~And make snapshots JPG's or something usable! Really, Nintendo, ****!
Audio
~Less orchestra, more rock.
~Same level of variety.
~Better F-Zero songs.
~...what happened to the DK Rap?! Put that back in!
~Be able to use songs on your SD Card to add to all levels (but you have to add them; it's not automatic.)
~More poignant sound effects (SSB64 level).
Roster
~Shorten it to 30, not including double characters (Zelda, Samus, etc.)
~Sonic and Snake, do what they want. Buff Sonic a bit to where he actually can K.O., and/or nerf Snake a bit and take out his infi-tricks.
~But Mega Man must be in this time, provided Megaman Scramble Battle isn't made. (I was going to suggest the TMNT in a Pokemon Trainer fashion, but we already have a TMNT fighter coming that's much like Brawl, so no need.)
~Ridley, for pete's sake. Just make a Bowser or Charizard-sized Meta-Ridley.
~Rosalina is a must. Luma is optional.
~Axe either Falco or Wolf, give the surviving character a Meteor Smash as instantaneous and useful as Melee Falco's. (See, now Falco has to be in just the right position to use that spike; wasn't the case, before.)
Gameplay/Mechanics
~No stickers.
~No convoluted Adventure Mode. Have an overarching story, just don't make it a platformer.
~Keep co-op.
~No more ridiculous achievements or Trophy Stands.
~Buff the Shield. Yes, it's helpful against Metaknight, but I've had enough of trying to shield against attacks, then they still work. Shield cancels all until it runs out (hence, why even bother having the shield breaking in the first place). This will also give Heavy characters who already have a large/better shield the edge they apparently still need.
~Give all characters a "Counter", perhaps as a result of Perfect Shielding. Replace "Counter" specials with other moves. EDIT: WHOA, and if you do that, increase the timing window on the Perfect Shield.
~Increase the range of grabs done by hand.
~Change. Ganondorf's. Moveset.
~Buff Mario!
~No Demos, okay? Thanks. Use that space for something, y'know, USEFUL. Leave VC demos to the VC.
~Very Easy, Easy, Normal, Hard, Very Hard. End of story.
~And don't flood the 1P mode with Giants, Teams, and Metal characters.
~Axe Home-Run Contest; Revive Board the Platforms. Make them character-specific, and only passable by knowing each character's advanced movement techniques (for example, Pikachu crawling under some hazard, or Sonic using Homing Attack on a target waaay out to the side).
~And, as always, remove the infinite loops. Metaknight, Sonic, whoever's got 'em, take 'em out.
~Same for chainthrowing. Ice Climbers, Falco, etc.