I agree with zlxq3000 about cookies kill you's idea. Although if it existed Beldum would be my canditate.
We really don't need new Pokeballs. Maybe a Light Ball to have the opposite effect of a Dusk Ball or a Luck Ball with a random chance of catching Pokemon.
Might be confusing, as the Light Ball is also an item that ups Pikachu's attacking stats.
shiny ball.
you run into a shiny that might like run or something, just throw it, and it's caught!
i coulda used that...
Now THAT'S an idea!
Weather ball-powers or decrease moves depending on the weather conditions
Already a move with that name, so there could be confusion. Nice concept, though.
My ideas:
Suffer Ball: Enhanced catch rate on Pokemon without status conditions. If it fails to capture the target, inflicts either paralysis or sleep (never confusion, poison, or burn). Will never attempt to inflict a status that the target is immune to.
Mirror Ball: Enhanced catch rate if your Pokemon and the target are the same species, or if one of the Pokemon is a Ditto transformed into its opponent.
Burglar Ball: Enhanced catch rate if the target is holding an item. If it fails, it steals the item and gives it to your Pokemon. If your Pokemon is holding an item, it puts it into your bag. If your bag is full, the item vanishes.
Mix Ball: 100% catch rate, but only for Linoone. Otherwise functions like a Great Ball. Name taken from the Mix Elixer item from Mystery Dungeon Sky (like a Max Elixer but it only affects Linoone).
Soap Ball: Enhanced catch rate if Mud Sport is in play. If it fails, removes Mud Sport's effect.
Sponge Ball: Same as Soap Ball, but for Water Sport.
Grounder Ball: Enhanced catch rate if the last move the target used was Charge. If it fails, it removes any of Charge's effects from the opponent.
Persist Ball: If it fails, it tries to capture the target once more. Will not attempt to capture the target more than five times, and no fewer than 2. If your Pokemon has Skill Link, the Persist Ball will always make five capture attempts. Catch rate of Great Ball.
Dusquick Ball: Enhanced catch rate if used within the first 5 turns of a battle. Also enhanced catch rate if used anywhere at night, or in a cave or forest anytime. If both these conditions are met, the catch rate skyrockets.
Brittle Ball: If it fails, it breaks and the shards damage the target, taking away 1/2 of its current HP. If the target's HP is at 1, it will faint.
ArmageddonBall: If it fails, all Pokemon on the field will have all their stats minimized, including evasion and accuracy, and will be afflicted with Ghost-curse and Perish Song.
Mystic Ball: A strange Pokeball carved from rock found in an ancient relic from long ago. The in-game description will imply that its special effects are unknown. In reality, it has an enhanced catch rate for legendaries.
Rescue Ball: Vastly enhanced catch rate if any of the following afflictions apply to the target: Burn; Poison; Bad poison; Leech Seed; Perish Song; Confusion; Ghost-curse; 5 or less PP for all moves (thus in danger of Struggle); HP in red and in possession of a move with recoil; Has PP for Selfdestruct/Explosion/Roar/Whirlwind/Teleport, and nothing is in play to stop it. Catch rate is 100% if any of these apply to a wild shiny.