Yi Buff that relies on proper skill usage (Meditate and Alpha Strike in paticular)
I play Yi since about 4 Years, im platinum and have over 500 Yi rankeds in S3, i think there are 2 options to engage more "play" in Yi without making him devastating in lower fields of play.
Option Nr. 1 Grant Meditate CC Immunity for the first 2 seconds WITHOUT allowing him to activate while CC´d. This would give him a on-demand defense for a short amount of time IF he activates BEFORE he gets CC´d. I think it its not as "Devastating" to give him a defense ability that requires him to channel rather than making him spent 80% of the teamfight untargetable.
Option Nr. 2 Removing the cast animation from Alpha Strike to remove the untargetablity BUT making it instant and give Highlander the old 18 second refresh. This would emphazise an "Assassin" playstyle with going fast in blowing the target up and using the mobility from the reset to switch targets immediatly while giving teams the ability to counter-react because he doesnt do it untargetable anymore.
A small add on itemisation: I think a Melee Carry/Assassin hybrid would work best if the item Sword of the Divine gets changed, it has high potential but has a big flaw because it gets LESS cost effecient if you buy items that are supposed to INCREASE its effectiveness and it also screws up your build because buying any crit makes the SotD active less effective. I would recommend to change any left over crit to AD during its active.