I get to post my list now, what fun.
- I personally like Brawl's physics, for the most part. I don't, however, like how much you slide around when your size scale is higher than two. Doesn't make sense to me...
- Dr. Mario should be an alternate costume like how Wario's anti-plumber costume is... It's called an "anti-plumber" costume, right?
- Moar Paper Mario! Ahem... Yeah, I was a little disappointed at how there were so few Paper Mario references. Why couldn't there at least be a song from any of the Paper Mario games?
- Leave the Debug Mode in the game so hackers can have fun with it... 
- Have more references to recent games. Did Brawl have anything from Super Mario Galaxy? I don't think I even saw a sticker!
- It's been said before, but improve the Stage Builder. And, I guess fix the Wi-Fi too, people seem to complain about that a lot...
- The Stop Watch item should "subtract or add X amount of speed to character," not "force character speed to X." The Stop Watch item is useless in slow-mo matches if they do the latter.
- All characters from past SSB games should return. If they would be clones, make them "alts." of the main characters.
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At least 6 colours for every character. And maybe make the Shadow and Emerald versions of characters unlockable?
- If the next story mode is going to be like SSE, all characters should have an important role. I hardly found any priority in playing as Ice Climbers except in the "Glacial Peak" stage.
- Classic Mode should be more classic. You know, a Race to the Finish, Board the Platforms, individual Target Tests (or smashes) for each Character, and fighting the "Fighting Based" team.
- Return of Red Shells? I liked those items...
- Since Kirby and others can eat items, shouldn't they be able to get special, temporary power-ups if they eat a certain thing?
- This is something minor, and probably doesn't even affect much, but if an area is unused and has no way at all to be normally seen or even accessed in game, that area shouldn't even exist. It really bothers me that developers leave graphics, and maybe even collision, in places that can't be accessed in any way. Also, how about actual sky boxes instead of a big "Billboard" like object? It looks pretty bad to me in Home Run Contest.
- A feature that lets the player add their own music into the game with an SD Card. That's definitely easier than using a PlayStation 1's music feature, replacing the Wii's Red Audio wire with the PS1's Red Audio wire, then changing the Music to Sound balance so that only sound comes from the left speaker and nothing coming out of the right.
- More hard coded Replays. If the game has a rare glitch on the Replay, the Replay playback will have the Human players continue to follow button presses, but the computers will break away from the replay and act as if it's a normal match, causing the replay to have odd effects. An easy fix could be saving replays as an .avi file maybe?
I'm forgetting some of the things I want, I'll post more later.
EDIT: By "More hard coded replays," I mean the Replays shouldn't be normal matches with the Human players following recorded button presses and the computers following a path of actions that gets overwritten if something in the Replay is changed.
