Been playing a lot of kayn mid and I've been seeing its potential as a powerful pick, with rotation/cleanup as well as insane wave clear with the mana cut costs for kayns q,w.
Keep in mind this is not at all a meta pick, and like kayn top, while their are free matchups, kayn mid is an uphill battle until form.

As a kayn jgl otp, I'm still pretty new to kayn mid, so i'll be updating this pretty often as I limit test more and more.

Ill explain the build real quick.
I LIKE FAARIE CHARM, its great for mana sustain, and I sell it after form. Its only a net 60 gold loss which is NOTHING for what if gives you imo.
For RHAAST-
Hubris is really really strong right now, infact its solid for both forms.
With all my experience w Rhaast, lethality is pretty much his only build. Bruiser form doesnt feel amazing, I typically only go one health item (usually cleaver)and the rest lethality. He's just not the draintank he once was :(. That being said there definetely are games where lethality/bruiser is the way to go, enemy tank heavy comps, or bruiser heavy comps.
Axiom is about the best thing you can build w Hubris on BOTH forms. You dont really need health early, esp as red(unless tank or armor heavy comp), so embrace the dmg/ult cd reduction!
Black Cleaver is the best 3rd item, its around the time where health becomes important, while having the dmg to back it up. Rush 2nd into tank or armor heavy comps.
The Rest is situational/preference. I'm very agressive after form, so I love to go Edge of Night 4th, but endless is good aswell!

For Blue-
Hubris
Axiom
Edge OR seryaldas depending on enemy items. whatever you build third build the other 4th. Only expection to me is when an enemy team comp has no cc (no edge) or no armor (no sery)
I like GA as blue, esp if your the one whos carrying. But if you feel like your team is good enough go bastion and split push!! You're a really solid 1v1 champ after form and have a great escape kit. TAKE ADVANTAGE OF IT.