Make the game interesting again!
Make armor and MR purchases strong enough to make a difference. That way players can decide if they want to build damage resistances or just go for the 100-0 nuke builds.
Right now armor, hp and MR just can't compete with the raw damage that every single champ has.
So what if surviving an assassin is op, Everyone wants to be op, and not getting 100-0'd is just healthy gameplay where skill, multiple rotations and strategy actually matter!
We would also have no need to lower damage then, because players could make that decision if they wanted survive-ability or damage! Right now its just do I pick this damage item, or that damage item.