When Kayn transforms he could let out a global voice line
A nice QoL change that I feel like would just add some form of clarity on what you should be prepared for, instead of having no knowledge that Shadow Assassin and just run right through a wall and burst you down in a second, or Rhaast run right through a wall and burst you down in three seconds.
Just some of the usual voice lines you here when you first transform, like Rhaast's "Goodbye Kayn. Reaping will commence. Now!" or Shadow Assassin's "I am chosen! The Darkin stain lives no more."
And I say this not hating Kayn, as I actually love that edgy teenager and sarcastic murderous weapon. Just would feel like a nice QoL touch to him.
Although this is Riot we're talking about here, so they would never really focus on and add a QoL voice line change to a champion