How about different weapon skin unlocks for city sections.
Examples:
Friendly Fire's on the East Side unlock SMG and Pistol skins.
Friendly Fire's on the Central Islands unlock Rifle and Shotgun skins.
Friendly Fire's on the West Islands unlock Special, and Rocket skins.
Going with that same thought.... Rim Jobs unlocks and vehicles set in a similar way.
Get all of the Rim Jobs on the east islands and you get X vehicle set added to your inventory... bla bla bla
Similar for clothing stores...
Other idea's...
Super Jump distance upgrades to either towers or rifts (towers would be better), but you still have to pay for the upgrade (this just unlocks it).
Speed increase unlocks with different races (again this just unlocks allowing you to purchase it)
Genki weapon skins should be set to beating all 3 of the Genki virus missions.
Genki Homie and Murder time Fun time costume unlock after beating the 3 "Mind over Murder" mini games (Silver gets the costume, Gold gets Genki Homie).
And So on and So Forth....