First of all: Why not just use Toon? Toon is about what it's like being in a Warner Brothers cartoon. This is a game about adventure in a cartoon universe.  

Notes:

The "adventure party" format is perfectly suited to this. Players take on the roles of band members, and possibly girlfriends or managers ('the Ruben Kincaid' type position).

So the first thing players do is decide what the band is called, and have a sort of theme.  

Characters, and indeed the world itself could be one in which humans freely intermingle with talking animals. The Hair Bear bunch for example, or the Banana Splits and Jabber Jaws coexisted with 'regular' (though cartoonish) humans. So it's this vaguely defined 'cartooniverse' has to be sort of put forward so that players know what the world is like. But not more than a page or so of description. Something like Toontown or Roger Rabbit, but divided into zones- so an undersea zone, a space zone, etc.  

So the next thing they do is decide whether they want to play any of a number of different types of characters. My initial idea is to allow characters to be:

What your character is in this game:
Human, Animal, Alien, Robot, or Object - this last could be something like a magically enchanted instrument-character or something.

Each of these has to be a significant design decision on the part of the player, and give certain bonuses or abilities that the other types don't get.


What you do in the game:

Solve mysteries/Fight (or at least 'foil') bad guys  

So it needs a "bad guy plot" system