Well, getting them to Riot's design team is a very tall order, and if it were easy, there would be a million suggestions flowing in every day.
But it's great that you have new ideas. Just post under concepts and creations, and describe the ability in great detail.
Example:
Q: Rebounding Bolt
Cooldown 10/9/8/7/6 secs
Fires a missile of electricity in targeted direction, stopping and dealing 70/110/150/190/230 (+ 60% AP) damage to the first enemy champion struck. It rebounds off of terrain, and will fizzle if it reaches max range or rebounds more then 3 times.