Well other than
Kennen, I think you're right. League is lacking in the electric champion department. Though what you propose is interesting. I think it would be cool to see a champion based around magnets or magical magnetism. Just throwing ideas around but I think this mechanic would be best suited for a melee tank top lane or support. Which is different because I haven't seen many champions that exist healthily as both a support and a solo laner.
I think the kit will need a passive or ability that adds a mark or stack to either an enemy or ally to "magnetize" them. Maybe an ability that pulls in attacks instead of champions to make them a better front liner. It could even be their ultimate, gaining damage reduction and redirecting all basic attacks or abilities to them.
Offensively they could, like you stated apply stacks to an enemy and consume the stacks to push/pull an enemy to deal damage scaling off stacks applied. This is good for balancing since you could always tune damage around how difficult it is to apply stacks and keep them there. Though this route would be mechanically complex and would probably need a range indicator similar to rumble's ultimate. Giving the player the option of selecting a target and then a direction for the knock-back. Otherwise you would need one ability to push a target away and another to pull them taking two ability slots. That isn't necessarily bad but it would remove a bit of depth and complexity from the kit. As for an ultimate I think an ability enhancement would fit best kinda like switching to stronger magnets. Increasing ability range, damage, and reducing cooldowns to allow for more spamability.
Overall I believe your concept is solid. You managed to find a niche that is pretty empty and a really interesting mechanic. I think your next step for further fleshing out your idea would be deciding on a role, then strengths and weaknesses. Once you have that deciding what passive and abilities you want to add into their kit will become a lot easier. Love the idea and hope to see you build on and complete it. Best of luck.