Just a quick thought I wanted to share.
Sometimes a champion deserves a balance change even if he is at 50% winrate (which should be a perfect state), a buff if above 50% or a nerf when below 50% simply due to the ratio of fun and balance.
I just realized that no matter how strong Irelia has been or how strong Kayle was, I never felt it was unfair for me to play against them. They both have telegraphed abilities that can be dodged and their kit is both fun to play as and against. Both champions had quite high winrates but they never felt toxic to play against, so I never banned them. I think this is definately the direction to go for future reworks.
On the other hand I feel like plenty of champions (my own mains included) feel extremely toxic to play against even when they have a subpar winrate.
I think biggest representative of this group could be Yasuo whose windwall will always be detested even if Yasuo has 30% winrate. It just does not feel fun to lose your entire combo to a single ability without it doing anything.
So that is about it, I think the balance of the game should be about fun, not winrates.