With the unveiling of Pokemon Trainer I remembered two very unlikely, but possible, characters.
The first is the Mii! Here's how it could work. You know how in Wario Ware Smooth Moves you assigned a Mii on a save file along with a name, same way with the online portion of Mario Strikers? Well, in Brawl when you start a new save file the game asks you to select a Mii and put your name in. The unlock able Mii character is then based on the Mii you assigned on your save file. It could work...
The second character are the Ouendan/Elite Beat Agents! While I still see them being assist trophies (my bet is that they won't appear at all
) they could have all three members at the same time on screen, while you control the lead cheerleader. The other two could help out executing attacks and the Final Smash would consist of the three of them doing one massive attack!
Its all wishful thinking, but the Pokemon Trainer opens up opportunities for weird and unique characters like the two I mentioned.
EDIT: Oh I just thought of a Final Smash Attack for the EBA/Ouendan. When they pick up the Smash ball you hear lots of people clapping and chanting "EBA". As they do the EBA are filled with the energy of cheering, music and dancing. When they are completely charged they release a massive beam of energy, like the one that destroyed the aliens at the end of the final stage of EBA.
Right now it sounds exactly like Samus's Final Smash, but they could alter it so that its pure EBA.