The next ten 'best character choices' are coming tonight, but here's some other ideas I had.
In a way, this system would replace both trophies and the coins used to pay for trophies. I still want them to include some sort of museum/information system covering all facets of Nintendo history, but using trophies again would probably include many rehashes and feel somewhat cheap. In another way, though, this is a different beast all together.
Basically, the Smash Medals are like Xbox Live 'Achievements'. Each time you beat one of the one-player modes with a different character, or play X amount of Vs. matches, or unlock something, you get a Smash Medal. To make things more interesting, they could make it so accumulating a certain number of Smash Medals would unlock something. You, and other people online, can also view your Smash Medals and see what progress you've made in the game. It would be a great way to encourage players to play every mode in the game.
New All-Star Mode
In Melee, All-Star was unlocked after getting every character. That makes sense, since you needed every character in order to give the mode sufficient length and challenge. However, there could be a way to include All-Star from the start.
Let's say Brawl has 35 characters (a good solid number, I'd say). At the start, you'd have 18 characters. With those 18 characters, you'd have All-Star, but only the first level of All-Star (which would feature only the first 18 characters and would likely be the easiest). After unlocking 8 more character, the second level of All-Star would open, and would feature those 26 characters and be more difficult. After unlocking all characters, the final and most challenging All-Star level would be unlocked.