Mecha Darkin
Mecha Aatrox is one of my favorite skins. The theme really works with his design. When Kayn was first teased, the first idea I had was how bad-ass a Mecha themed skin would look like.
Mecha Kayn could definitely work with his base look being based on a Pilot and Rhaast on the foreboding Jaeger-looking Scythe-wielding Mecha Darkin form. His Shadow Assassin could then look like a PROJECT skin where his base form gets augmented ala Gray Fox.
Would be one of those skins that would translate quite well in theory