Crazy-ass but also appropriately thematic buff to Nocturne

Okay, so I saw a fair amount of discussion recently about Nocturne and his weaknesses. He used to be really powerful and widely picked, but his tools are no longer impressive or super unique in today's jungle arms race. In light of the Juggernauts getting more specialized roles, I thought of an idea to give Nocturne a more unique identity:

What if the shadow trail left by his Q let him pass through terrain, instead of just ignoring unit collision?

Not only would this fit his theme as a spooky ghost by being able to walk through walls and surprise you, but it would also contribute to his identity as a chase-based character. He already has tools to dash across the map to a target, and his kit encourages him to stick to targets for extended chases by letting him ignore unit collision and giving him delayed crowd control. Having a weird tool that lets him chase enemies through walls, so long as he's already marked them, would add to that identity.

Nocturne would still have to be targetable while within a wall, to prevent this ability being used too defensively by ducking into a wall and avoiding damage. Maybe it could have a really long cooldown that gets reduced when you hit with it or perform basic attacks while standing (or hovering or whatever Nocturne does) on the trail. You could fire it through a wall and pass through to gank someone, but it would mean the ability was on a longer cooldown since you didn't use it offensively to chase.

Also, being able to create private paths through walls would probably be overpowered, so Nocturne probably shouldn't be able to attack or use spells while inside a wall. Maybe he could have gray misty particles around him to show he's in a "smoke form" and vulnerable while phasing through walls, although that might make him look like he's harder to hit, instead of weaker. The idea, though, is that you shouldn't want to stick around inside a wall, since letting him fight while phased into a wall like he's mid-Magical Journey would give him much too powerful of an advantage. The trail would also still work the same way regarding blinks vs. dashes, so if Shen dashed through a wall while marked, Nocturne could chase him through the wall, but if Ezreal blinked across, the trail would end on one side.

I know this would contribute to mobility creep and take away some of what makes Bard so cool, but I don't think it would cut into the Caretaker's niche too much, and it would most importantly give Nocturne something really different to make him stand out.

Plus, you could type "BOO" in /all chat after ganking someone through a wall.