Shoot. I was going to say "The roster", but eh.
I love the Adventure Mode, but it COULD use a bit more spice... Once you realize it's nothing more than a huge Kirby adventure, it... doesn't sit well with you anymore.
Alas, I'd probably fix the overall mechanics/physics of the game. In Melee, when you threw a punch, it lasted for a little while and sometimes the characters would get the "aftermath" of the hit. It wasn't as powerful, but it still hit 'em. In Brawl, when you throw a punch, it's like you miss them that ONE frame and you're done in.
Plus, there's also the brokeness of Meta Knight and Snake to consider.
And poor Captain Falcon... His Falcon Punch has lost a lot of power/knockback from the transition of Melee to Brawl.
And how you latch on to an edge when your five feet away from it. Seriously? Melee, you had to be RIGHT next to it. That's stupid.
All in all, I'd make it more Melee-veteran friendly as opposed to ... what was it, casual gamer friendly? n00b friendly? Something along those lines.