I agree that Nintendo's multiplayer-centric games really need an arcade/everything unlocked mode. Just make it so that, when you're playing with more than one controller physically hooked up, you can select anyone.
Call Of Duty 4 on the Xbox 360 has an excellent method for this. When you're playing split-screen multiplayer (which is local only, no split screen online), you can choose one of a ton of different classes, all of which are very well balanced. Instead of forcing you to get to level 55 to unlock every bloody weapon and perk, they just give you a bunch of intelligently made presets (all of which are way better than the 5 themed classes you're given when you first start playing online multiplayer) that anyone can select. If you play online, you have to earn those perks, but there's no requirement that you unlock stuff if you want to just buy the game and bring it to a party.