I had an idea for a house rule using the Rookie, or any of the additional GB if you so choose, when you are not using him as a regular playable character.
"Let the Rookie In"- this rule puts in the Rookie in a trainee capacity, similar to how I imagine the allies might work on the GBII Get Real add-on.
Flip his card over to his biography side as he does not get any of the normal gameplay abilities, but you will still want to have his card out to hold trapped ghosts.
Rookie is a 5th GB who is always in play and can be used by any GB on their turn.
Rookie is allowed 1 action and 1 maneuver per each GB turn.
He can attack and add streams to ghosts, but no extra XP is awarded to players for the streams Rookie contributed, essentially he's a spare combat roll in a pinch, but you don't get extra XP for using him.
Rookie can NOT drive the car, but he can go inside of it as well as deposit ghosts.
He can get slimed and lose his one action until de-slimed.
I think he'd be a good addition for people who are looking for a little extra help in a scenario.
I played with the idea of adding sidekicks to the game with my custom Mystery Inc character cards, which would have had Scooby behaving similar to your Rookie idea, except that a player using Scoob Doo on his turn had to give up one of his own character's Actions to give Scooby an action (except for Shaggy, who got to command 1 Action of Scooby as a Free Action at L2). I eventually just made Scooby a separate character so he would have his own level abilities.
Of course, the new game has Allies, which may full this kind of role.