Highlander should be "You are untargetable unless you used an ability or attack in the last 2 sec"
No attack speed boost. No movement speed boost. No slow immunity. Just the ability to be at the right place at the right time all the time everywhere.
The animation would look similar to alpha strike's shadow images, except he would perpetually leave them behind (they fade fast). The whole "moving so fast you can only see mirages" thing is Yi's design space, and fast doesn't have to mean he outruns a horse. It can mean he is nimble, agile. It can mean even though you can outrun him, you cannot touch him.
Besides it would synergize with meditate (survive for 2 seconds without auto attacking and voila, you are untargetable again). The cooldown of meditate is large enough so that even with 70% resets due to ult, using this trick is expensive and if he's forced to use it he will have to play more passively until he can get an assist.
The counterplay is to counterattack when he attacks, and to zone him off of people by threatening to blow him up the instant he becomes targetable. His gameplay revolves around using the perpetual untargetability to be in the right place at the right time, ready to make the one strike that will win the battle. This is more in line with Yi's identity as a blade master than "I just go real fast".
It gives him options in case his plans don't work out, and it makes him less oppressive when he is crazy fed (he loses so much dps and sticking power with this change). However, it makes him better on average in team fights because he has a way to break aggro and keep it broke. That means that he can afford to put more gold into pure offense if he is snowballing, which counterbalances the loss of damage in his ult.
It more or less becomes a matter of mind games, and mind games are more interesting than "yi has nuf stats, so he wins" or "yi has not nuf stats, so he loses".
What I'm saying is the gameplay for this is deeper than it is for slow immunity and high DPS.
