One thing I think Kayn could use is the ability to track which form is "winning"
I'm not entirely sure how it works, but it seems as though the "winner" of the struggle is determined by what kind of champion you attacked last before the circle is filled up. In my second game as him I tried to go for Rhaast so I ganked top lane and counterjungled a bunch to meet the only melee enemies, but the circle was filled with the orbs I got from attacking the enemy ADC so I ended up with Shadow Assassin. I'm not sure how it'd be done, but I think some clarification on how this works would be helpful.