How about adding a bit more depth and complexity to his kit by having Windwall tied to his Flow mechanic? You still gain flow and still have your shield, but your W consumes all current flow and stops flow during its 3.75 second duration. It makes sense to me thematically, thinking of flow as the wind and the Windwall and passive as Yasuo's focused flow.
If Yasuo makes a mistake and finds his passive popped, he can still Windwall but doesn't have flow for its duration, lengthening the window in which Yasuo is vulnerable to an all-in. A good Yasuo can still stop two instances of damage through timing W after popping his passive but gives up generating flow for the duration of his W, allowing for counterplay as forcing a Windwall stops his passive for 3.75 seconds and eats his ability to shield. Overall a nerf, but changes an otherwise boring passive into something requiring a bit more decision making because of its relation to the use of his W.
Overall a nerf that really isn't needed as Yasuo is actually kinda fair, although snowbally if unchecked.
Edit 8/21:
I've been mulling this one over for a bit.
I wanted to build up on the Windwall <> Flow <> Shield interactions to make the passive mechanically deeper, rewarding a Yasuo for skillful use and timing of his abilities and punishing a Yasuo that plays sloppy, while opening up the passive to counterplay that's more interesting than popping his shield before you can actually deal damage. The abilities, resource, and passive would still have the same interactions outlined above but if you Windwall with your shield up, it takes your shield too. The two scenarios in my original post still works, but now adds a third case where if a Yasuo isn't thinking about when they are Windwalling, they're actually making themselves more vulnerable to damage.
This opens up even more counterplay options for opponents, as they could now bait a triggerhappy Yasuo itching to W into using it too early and now no longer having their passive shield for the 3.75 seconds Windwall lasts on top of the time it takes to get 100 Flow. This also gives a clever Yasuo the ability to play mindgames with opponents as now there's a question of whether or not Yasuo will Windwall, making the passive even deeper and more interactive, and also INTERESTING TO WATCH when spectating games.
The interaction between Windwall <> Flow <> Shield should also carry over to his Ult. Currently, his ult gives 100 Flow for a shield. With this proposed interaction, his Ult should also drop the Flow debuff (no flow for the duration of Windwall), meaning that a solid Yasuo can soak a hit with passive, Windwall, ult in to gain 100 Flow for a second shield plus the 50% armor penetration on crit, while dropping the no-flow debuff due to using his ultimate. This rewards skilled play by sequencing abilities correctly to maximize his passive's damage mitigation. This also means that if a bad Yasuo ults in, they just gave up at least one instance of passive mitigation and if they Windwall right after, they give up two.
First Scenario - Passive, Windwall (Flow debuff), Ult (drop Flow debuff), Passive = at least two instances of mitigation (excluding anything Windwall stops)
Second Scenario - Ult (gain 100 flow), Windwall (Flow debuff) = no mitigation (excluding anything Windwall stops)
Overall, I do like Yasuo a lot, but I find his Flow <> Shield mechanic as it exists now feeling like it was tacked on as an afterthought so that Yasuo could all-in easier with at least one instance of passive mitigation when he ults, and that's just really boring.
Thoughts and feedback appreciated.