'Forbidden' Design Choices

Dear Riot, I was wondering whether there are any mechanics that are just point-blank disallowed from existing due to potential balance problems? For example, have negative passives ever been considered to allow for power-gating, or the ability to permanently stack an affect on an enemy champion to be triggered later (in opposition to the decaying examples, like Rend stacks)? Many thanks, ThePianoTuner

PS. If anyone else has interesting ideas for unseen mechanics, please post them. Could be fun.