Yasuo is my most played champion right now, mostly because he's more consistently safe pick than playing my 2 season main, Aurelion Sol, or at least that's how I've felt for a while.
He's either very good or very bad, which I don't think is healthy. This results in him being banned a lot because people don't want to gamble on it. Well, that amongst other issues.
Another big problem I think is that he's quite strong alone; but if you have anyone with a good knock up on your team, his power grows exponentially. Malphite comes to mind. Maybe making him unable to R off of allies would be a healthier change?
I'm not really sure how they would go about changing him. I like how his Q is right now. Tornado from a sword swing is really cool. While it lets him stack damage really fast he has to push wave to do it, and frankly tornado is kinda easy to dodge. I feel like changing this you would want to look at it's range, cooldown, and damage. Maybe more emphasis on comboes like Ryze in order to make it less ...well, easy? Since right now it's just Q-Q-Fire. Maybe holding down certain buttons would change property of Q third stack, I dunno really. A lot of my ideas probably arent so balanced.
Windwall is pretty iffy. Can be a complete engagement changer, like how it blocks Ornn R or crucial stuns/slows etc or big damage like MF R. I feel like putting his shield passive into windwall would be better. Windwall consumes shield passive and blocks damage based on shield passive consumed before breaking, size and health of wall increases with level. Or maybe blocks a certain amount of hits like Yoricks wall.
E is probably what people hate most about yasuo aside from windwall. Unable to run away from him if you get minions nearby, and you have to be very careful of how close you are to minions during laning. Maybe add a limit to dashes, like 3-4 dashes that recharge. This way a yasuo needs to commit to a kill and cant disengage easily if they get cocky and bite off more than they can chew. So they wouldnt be able to dash up through a wave, if they see enemy team coming they cant dash back through the wave.
R is another big point of contention I think most people hate the tower protection most. I think personally it would be better if they turned it into a very fast dash instead of a blink, so you dont just mash R when you see someone getting knocked up by Q, so you have to aim it. Im not really sure what to do about the armor pen passive. I feel like it should be changed but I dont know to what. Maybe small amount of raw or percent damage of some sort but not 50% armor pen. Maybe even AoE basic attack while its up but Q is already AoE. I think more emphasis should be placed on it's damage instead of it's utility, but that's just me. But like I said earlier make it so it doesn't work off of friendly airborne. I know people dislike high damage game right now.