My ideal assassin (they are like bruisers, but have innate mobility rather than innate tankyness)

I think a good assassin is someone with the potential to 100-75% you and then get away without very much risk at all. Assassins should go well in poke comps, like with jayce, because if the adc gets poked down to 75% hp or below, then the assassin can one shot them.

I think assassins have very little counterplay and require very little teamwork compared to other classes. I think that if you take away 100-0 capabilities, but make them safer via escapes and lower cooldowns on mobility then they become balanced. They should have very low cooldowns on all abilities, but should require more than one rotation to one shot you.

They shouldnt be tanky, but should be safe due to cool escapes and stuff. They should find it easy to engage on a ranged target and deal dmg, then get away. They shouldnt do much more than 75% of max hp with one rotation (assuming they arent super fed).

Ap ekko is a pretty good example. His q e passive combo does about 75% to 80% of your hp and he can get away with the speed boost or with his ultimate. He doesnt usually oneshot you because his ult is 4 seconds behind and he needs that for a oneshot. The adc and the adc's team have a chance to recollect themselves and the adc can lifesteal back to full on minions, but they are weaker in the time period after his dmg is dealt bc the adc is at 25% hp. Doesnt cost much for ekko to do this. It costs him two 4 second cooldowns and thats about how long it would take for the adc to LS back to full.

So if ekko is good he can keep supressing the adc with his combo and go in for a kill if the adc cant LS back to full in 4 seconds when the combo is back up. Or if the adc gets poked to less than 75% by jayce nid or ezreal then thats a oneshot opportunity. Or if he is a god and lands the stun ulti combo. The counterplay is in the fact that you need to dodge poke or be able to ls back to full in 4 seconds or you need to stay away from his ultimate. All of that is pretty do able, but theres room on both sides to continue making plays after the assassin does his job and gets a momentary advantage for his team.

Unfortunately, ppl only build ekko tank and they dont see him played as this kind of assassin. It works. It really does. U just have to play based off of cooldowns and keep trying to get both sides of your q to land on someone.