Negative stats

Why not?

Wrote a big essay the first time and my phone clicked a link on my and erased. Basically make cross builds more of a tradeoff than they currently are by introducing an unfavorable interaction - namely a debuff that wouldn't hinder the class that the item is intended for and should be able to use to the greatest effect.

Something like:

%AD and AP decrease for items with Armor/MR + health.

Wouldn't effect base damages meant to be the primary damage for tanks, and it would curtail assassins with high base damages from being too safe with too little counterplay, without hindering their ability to operate as intended at all. If they did want to build defensively, it would need to be more carefully considered to be worth it.