If I could change Yasuo...

I'd make it so his tornado couldn't crit, reduce how long he could hold his tornado (thankfully the PBE is doing that) and make it so instead of growing in size his windwall would have an increased duration (obviously starting much shorter than it is currently) per rank.

Reasons: tornado crit is just a personal annoyance, no real balance philosophy behind this. The holding duration ties in with the crit in how I do not want to be hit by him, so he ends up zoning people off for very long. Other people with this (TF and Annie) actually have a weakness here because they can't farm as effectively while zoning. Yasuo doesn't however (course Yasuo is a different champion).

Windwall: this ability is probably the most accurate definition of one point wonder. It scales in size with rank, but a good Yasuo never needs a bigger wall. They'll just put the wall in the best spot because just a single pixel can delete projectiles coming at it. So instead, keep its base size and reduce its rank 1 duration to better reward skilled/reactive Yasuo players for using their reactive ability. Then as you rank it up, it increases in duration. Giving feeding Yasuo's some better back up utility to fallback on.

The windwall nerf at least hurts his level 3 early and probably a bit till his late when he finally starts maxing it.

That's just how I'd change Yasuo.