I'm guessing you didn't fully read my paragraph of analyzing the complaints given about her passive...
Whatever, but her previous identity was that she could use her AOE to her advantage to put some DoT onto her foes if she laid down the noxious blast(or whatever the spreading circle of poison is called), then they are slowed, and they have to back off because they have seconds of AP slicing off their health, and they either have to coordinate someone looping around the long way to catch her, or just give up and say "when she gets back to lane, we need to team up on her, and block her off so she has no way of escaping", since her only form of defense past her noxious blast is her ultimate, which only slows if her chasers are running away from her.
I feel like most of the "AOE mages" you mention, are those which have kits that give them the true identity that they needed, and it worked for them for the duration of them being on the live servers.
Ziggs is mostly AOE, but his kit has upsides and downsides to it: his Q is great for poking when enemy is under tower, his E is great for when the enemy team is clustered together, because they'll all be slowed and damaged with each bomb they step on, his W gives him an escape or an engage to lead into another team member's combo, and his R is great for picking off people who might be low, and aren't really paying attention to the giant red circle that means almost certain death for them.
What other AOE mages are you thinking of? Because I can only think of a few that really play almost the same way, but the point is: their kits all gave them some sort of identity that gives them variance from the rest of the large 123 champion roster.
Cassiopeia(I just thought of this now, so bear with me) should have a passive that adds a stack of poison DoT that increases each second they have are damaged with one of her abilities that apply poison to them, and stops at a somewhat reasonable number(5, maybe) and the DoT is a certain percentage of the total AP per second(10%?) or a base amount with a percentage of total AP and then increases the percentage added to the poison damage based on the passive stacks given to the target, and also adds to the combo-based kit that she probably has now, and then offers some sort of skill cap to allow positioning and proper sequence of abilities, rather than just spamming E because the target is poisoned and now you get to use E again in a second(change E to deal base damage, but maybe just continue the duration of the poisoning by X seconds, and maybe at a certain point of the game, the ability(either when maxed or current passive reaches a certain amount of 100 stacks -_-) gives the poison damage to surrounding enemies if the target is already poisoned, increases the range, and does something else to benefit the other stats she has(X% increased AP, for example)
Any OTHER Questions?
P.S. What I said earlier isn't at all saying we should return the jungle items to the confusing and misguided ones we had earlier, which didn't really give any real guidance as to which kind of jungler works best with said jungle item. I'm saying Riot needs to add some better gameplay paths to Cass than just power farm the entire game and hopefully maybe come out to a good contribution to a team fight at 30+ minutes