His Q is very nice, mana cost will probably go up
His W is the biggest worry, early game it takes much skill to land, but late game in big team fights it's easy to miss, but really, that's your fault not his :P
E is very nice, though often the range is confusing, (same with W) it feels like you're in range when you're not, especially with E and speed boost, it's weird when you "can't" catch a slower opponent with a double dash, but that's probably cause I'm just new to him
His R, this is where a nerf is most likely incoming. The idea of using it to escape or do damage is why it has such high ratios, and since he's a fighter, many will cross build him with AS and/or armour + hp. But if you full AP him, and watch your positioning, he wrecks team fights with Yasuo level Jukes. I have engaged, almost died, flashed out to run away then ulti back in for easy triple kill i think 3 / 4 games. (only played him 4 times) The damage on his ulti late game is too much, I love it, but I know it'll get reduced (or they will add some other weakness)
My Build so far:
Core:

Main
/
(depending on ahead or behind)
As you can see, not much magic pen or CDR, cause of his high scaling, deathcap should be in there, but his HP is SO low as a trade off. Looking at using Ludens Echo OR Lichbane, though they work very well together
Want to make room for:
and for tank builds 
I think if I don't use abyssal (or when it's changed) I'll work in
to help with his engages, and
or
and 
Once I am happy with a combination, I'll try him in ranked