Anti-Mage Champion Idea
I think it would be interesting to have an anti-mage champion. His story might be that he was good friends with someone like Syndra but he saw what magic did to her and wanted to turn her back or if that was impossible to stop her.
In his quest he found an ancient legendary sword designed to in times past by warriors to fight mages. Upon the blade was etched "The Arcane will be destroyed by their own power." It was created to use mana; not it's wielders mana but rather his opponents. It could strike harder, siphon it off, and even reforge itself stronger. He drew this sapphire hue rapier and felt a great power in the weapon. He had a weapon and a goal but lacked the skill to achieve it so he went to join the League of Legends to learn how to wield it for his upcoming fight. In the preliminaries everyone took one look at him and though it was a joke but the sword showed it's power impressing those in charge. The real legend is the sword, he is here to learn from it.
He's a support class who is able to incapacitate opponents. He has very low offensive stats and usually requires someone else to get the killing blow in but is able to weaken them, but possibly an above average attack growth stat.
Passive: Mana Strike, Grants basic attacks a damage boost based off your opponents max MP. Your mana pool is drained when you attack an enemy champion for bonus damage. Q: Siphon Rune, your next basic attack will place a rune on your enemy so there abilities do % less damage and % the mana they use is added to your energy pool. % increases with points put in skill. W: Shut it, single target ranged skill that if it hits slows and silences an enemy, can miss and can be intercepted. Cooldown reduced and slow increased for points put into skill. E: Mage Guard, Active: Create a shield that blocks magic damage and a % of the damage blocked is added to your energy pool. Passive: Increase Magic Resist. Magic resist, shield duration increase with points put in skill. R: Mana Forge, mana attack bonus becomes a temporary buff so you do the extra damage each hit against any enemy. Increases attack permanently by n (should be a low number) as long you have a minimum amount of mana. Cooldown reduced and buff duration increased with points put in skill.
This is a more Support Assassin style. Shut it, Siphon Rune, Mage guard is a good combo I think. Mana Forge also allows him to overcome his poor attack stat over the course of the game, but he has to earn this attack boost because of the minimum mana requirement, not just spam his ult. He's a late game champion who is only good at support in the beginning but can be powerful late game, like a weaker version of Nasus. He can also help in team fights by weakening the more dangerous fighters, a bit like using exhaust.
Strategy for playing is to get Mana Siphon and Mage Guard to fill your Mana bar and then use Mana Forge to start attacking. Shut it is used for escaping or getting close.
Strategy for playing against him is to avoid shut it, don't let him get close and avoid him when he does mana forge until the buff wears off. Also shift your focus while Mage Guard is to someone else until the barrier fades (like Kayle's ult), unless you do physical damage. Basic attacks are still effective. A build if you're fighting him a lot is to focus more on mana regeneration rather than max MP, and blind's are extremely effective on him (Just like Udyr).