Mario & Sonic Guy
Old rivalries live on!
Maybe for one secret message example...What they could do is much like the stickers (but at a CD rarity level) have notes appear and you pick them up in battle. After wards you see a little notepad that tells you game secrets. It could be unlockable character hints, challenge hints, stage unlocking hints, or Easter eggs.
"The Mario Universe roster is almost finished. Conquer Classic Mode with what you have to get someone special."
Basically, the hint would become available when there's only one more Mario Universe character to unlock. Accomplishing the task that the hint offers will offer you the challenger match. For one unlockable input, utilizing this hint...
Rosalina: Clear Classic Mode as all of the other Mario Universe characters; Mario, Luigi, Peach, Bowser, Bowser Jr., Donkey Kong, Diddy Kong, Dixie Kong, King K. Rool, Yoshi, Wario.