Rather than just nerfing Quinn please do this instead
Quinn is one of my favorite and most played champs and I've had this idea for while which usually gets a good reception from the community when I share it.
Put her ulti to her passive and her passive onto W:
Make it so when Quinn walks in a single direction without making any hard turns Valor swoops down and lifts her up increasing her movement speed. If Quinn makes a hard turn while in flight, Valor flies off and goes on cooldown.
This will make her speed buff require skill similar to maneuvering Sion ulti. The way it currently is she can make a hairpin turn at 700 ms and not lose anything for it. It's ridiculous to look at on top of requiring no skill. I'm not complaining because I still like to play this champion but if anything had to be changed about her I'd rather something like this than moving around the damage numbers.
For her ulti the sky's the limit. Riot can give her some kind of damage ability or some weird new interaction with Valor or something that gives her brief untargetability. I'd be open to any of those and I think they could all work.
But please don't just give her a boring damage reduction that makes her unviable for months until she gets another random damage buff down the road. I don't think that's what anyone wants.