Assassins should be balanced around dealing 2/3rds of a Champion's HP with one full spell rotation.
HEAR ME OUT.
I've always felt like the class should be more poke/duel and fight really hard for that first 1/3rd of the Enemy's HP with Basic Abilities and Auto-Attacks so that you can use a Full Spell Rotation (including ultimate) to deal the last 2/3rds. SO many Assassins are so binary in their design in that you just mash all of your abilities on one person/location and hope it all lands. And with the advent of Thunderlord's Decree which simply dictates that if you hit someone 3 times with anything you get a whole heap of extra compound burst it's even worse.