Here are some ideas:
Vehicles:
A car/Jeep/Plane that takes damage when drops from a height or flys into the wall at a high speed.
A car with a gas tank on it that make the whole car explode when hit by a bullet.
A car with a gas
meter that you have to tank at a gas station when it's empty.
A car with a radio, that can play music add-ons (over asked, I know. xD)
A car with front and back lights.
A car that goes bouncy (I don't really know what it's called ^^)
A car with nitro.
A car with a trunk, which you can throw stuff in with CTRL W, and let them hop out by pressing on the trunk.
A Vehicle that drives really slowly on ground, but goes fast on water.
Weapons:
A heavy-but-powerful sword that takes more time to attack with,
A gun/snipe that shows how much bullets it has on the top or something,
A Boomarang that comes back after thrown
A Baseball bat. When you hit someone with it, he'll fly for a short distance. Kinda inspired by the SSB series.
Mini-games:
Vollyball?
Scripts, emitters etc.:
A paint can that turns (for an example) two 1x1 bricks standing next to eachother, into one 1x2 brick. (would lag less)
A flower emitter.
Links to finished add-ons:
(none yet)
Anyway, thanks for reading. Whoever makes one of these add-ons gets C

kies :D