Yes, the passive needs something AoE to make it comparable to other damage based passives (i.e. Ori's). Well, lets see, if we make the passive so that the AoE affects all enemies, then if we use an AoE on a minion wave + enemy, then trigger the passive with another AoE, the ability will be completely overpowered. Simultaneously however I think the true damage helps farming, so I don’t really want to lose it on minions.
Solution: Marked enemy Champions w/in 400 units of each other are affected by the strong magnetic interactions between their two marks.
Now there are a number of ways you could take the ability from here; it could be straight damage output based upon the number of enemies w/in the AoEs, or chained AoEs, or it could have a cc effect sort of like Kennen’s passive. Since the rest of the kit has a lot of cc, lets go w/ damage.
A straight damage granting passive would be ori’s, it does 10/18/26/34/42/50 +15/35/55% Ap scaling per auto attack. An AoE should do less single target damage than this per rank for 2 champions hit (since single target is something else), do equivalent damage at 2-3 champions hit, and better damage for 4-5.
Your AoE passive ability can proc (w/o cdr) at most once every 12 seconds. Ori’s base attack speed starts at .658, and ends at 1.07 w/ no AS items/buffs. Lets average this to be about .8 attacks/second. Lets say that she can’t attack uninterrupted the entire time, so she gets 4 attacks off, then the stacks on her passive degrade, she stacks the attacks again, and so forth. Her first round of attacks takes 3.2 seconds, say there is a 4.8 second pause, her second round of attacks takes 3.2 seconds, and say there’s another attack at max damage thrown in randomly in there so the total time is 12 seconds. Total base damage is 90/180/270/360/450 + 3.75 AP scaling. Balanced base damage should be about a 25%-33% of this amount, so the effect should do between 15/45/68/90/113 and 30/60/90/120/150 + .9 scaling.
Well, this balancing assumes you only use your abilities to apply the charges; considering you could also auto attack to stack charges these values should probably be reduced a bit. The damage from Ori is probably a bit of an overestimation as well, so I’d tend to scale these values down. Maybe .5 AP scaling, and 20/40/60/80/100 damage.