I am sorry to say this, but I disagree with the premise of your post. There should be no rules. Only practicality. If a champion is going to have a unique set of abilities, the uniqueness should be focused on, and other abilities shouldn't distract from it. This is practical because 'overloaded' champions tend to be OP or painfully complicated.
For instance: Yasuo is broken because the crit on his passive is a simple stat-breaker. Who else can get 70% crit from a single item? No-one. And yet that is not the unique part of his kit. He has a unique passive 'flow' but that is not the unique part of his kit either. His Q is unique in that it works like and auto and it synergizes with his E and R, so I argue that his Q + R is the unique aspect of his kit. The crit adjustment is a distraction from that interaction and his 'flow' interacts with his E. Not bad per se but a little overloaded.
Illaoi has loads of unique interractions with the tentacles, but the worst one is the 5% missing health passive. Since it doesn't scale off of bonus health, and she has such high AD Scaling, it encourages Illaoi to build damage and take risks, hoping to live from the restored health. This is a subtler issue of being 'overloaded' caused from a discrepancy in her design; it's as if she were designed to be two things, and instead of committing, they simply added as much of each into her as possible. The same goes for Darius, they each have too much damage for their intended role: Tank/Juggernaut. While they should be able to deal damage, they can both get away with dealing damage and not reaping the consequences of a reduced survivability. In this way it is a game-rule discrepancy, Riot designed the game to have certain roles with corresponding playstyles. That's why most tank items don't have much damage, and most damage items don't have much survivability. The recent changes do seem to be breaking that rule though with Death's Dance, Phantom Dancer, and some changes to tank items--and I'm not sure that's the worst direction to go in. Though I can't say if it's the best either.
An example of a passive that takes the focus properly is Kindred. Their passive is the focus of their playstyle, and the only other unique aspect (I argue) is the wolf interaction with the W and heals. Everything else is simple enough not to be a pivotal piece, and therefore takes a secondary place. Therefore you have an excellent passive that comprises the majority of their playstyle, and is not distracted from by the rest of their kit.
~CrimsonCobra3
Passive's should compliment the abilities of a champion, not the other way around 
When a champion dies, it should stay dead 
Once a champion is dead, they should not be able to do damage 
Abilities can be offensive or defensive, but should never be both 
