While in the shower, I was thinking about what could be a pretty fun idea for the DS/Nintendo WiFi Connection.
It's a two player game, the first player playing as Mario in what would appear to be just a regular, ordinary Platforming game (a la Super Mario Bros., et al.) where Mario must save our fair princess from the evil clutches of Bowser. The Top screen displaying the actual game, the bottom screen displaying some various stats for...
The second player, who plays as Bowser (or "Bowser's Army"). Your job is to keep Mario away from the castle at all costs. You'll have control of a limited army (for each level) and be able to pick and choose where/when the enemy is deployed. You'll get a few seconds head start in front of Mario, placing your enemies throughout the level (although there'd have to be limits - like you can't place enemies on the screen that Mario is currently on).
Now picture this - the top screen shows Mario jumping from mushroom top to mushroom top (like SMB1 Level 1-3). On the Bottom screen, the second player gets to choose the level and frequency (of course, the cannon would need to be reloaded and moved up and down!) that Bullet Bills get fired out at the first player.
Think this would work?