We already have tournament rewards in the form of avatars and cold, hard cash, but I've been exploring ideas for other types of events, like a TMM Blitz to boost TMM numbers, or for the various ladder leagues.
Stuff I've considered so far:
Money: Covered by tournaments
Avatars: Not particularly "special" since we already have these in the hundreds
In game cosmetics like ACU skins or hats: Would be awesome to have down the line but requires a lot of dev time to implement
Colored (or with effects) usernames in client: Would have to override or completely remove the ability to do so by individual users