I wanted a champion that was not so easy to play that a monkey could do it, and had their own unique flavor; so here we go.
Lillith is all about risk management. You see, she's a squishy, but she also needs to get hit. She also has no (obvious) direct attack spell! But you know what? I bet she'd be insanely fun to play. Lillith would be murder against any spell casters, but would be rather helpless against any melee oriented champions.
Passive: Learned, All teammembers get 3% bonus to all spell damage
Q: Unstable Sheath
Lillith enhances an allied unit with shield that will absorb 5/10/15/20/25 percent of incoming damage for 5 seconds. When the time has elapsed the sheild breaks apart, shredding nearby units for 100/162/225/288/350 magic damage.
W: Runes of Warding
Lillith is surrounded by runes of warding that increase the magic resistance of her and nearby allies. She may choose to disperse the runes to deal damage to surrounding enemies, and decrease their magic resistance at the cost of the aura for a short time.
Passive: Provides 10/22/34/46 bonus magic resist
Active: Lillith deals 80/120/160/200/240 magic damage to nearby enemies and decreases their magic resist by 10/22/34/46
Lillith casts a projectile that, if it hits, curses the target with bad luck. For 10 seconds all the target's spells have a 5/9/13/17/21/25 percent chance of backfiring horribly. Backfiring spells act as if they were cast by Lillith, using her AP as a modifier, and can target any of the target's teammates, creeps, himself, etc. in range. Jinx! Is removed if a spell backfires.
Ultimate: Mind Leech
Lillith has a 10/15/20 percent chance of learning the last harmfull spell used against her. If you already know a spell, it is overwritten.
Lillith casts the stolen spell, using her own AP(x1/x1.1/x1.2) as a modifier.
Lillith's current stolen spell is forgotten.
The champion that owns said spell has it go into cooldown, or has the cooldown refreshed, and is stunned for 1/2/3 seconds.