Most Quinn mains are perfectly aware of this.
Her biggest problem is her bugs. This isn't news.
But a mini-rework would probably be welcomed. Mainly on her ult. Make Valor more assassin-like.
Passive - Valor form now reduces the cooldown of basic abilities by 1 second per auto attack. Getting a kill resets Quinn's basic abilities when leaving Tag Team.
R - Damage removed. CD reduced to 45 seconds. Duration removed, but the MS will no longer apply after 20 seconds. Available at level 1 and does not rank up. Abilities have 6 ranks. If Quinn activates while targeting a target marked with Harrier Valor will dash to the target, consuming the mark. If Valor activates while near a marked target, Quinn will land a basic attack on that target as she comes down. This will not apply the Valor kill reset. CD does not go below 25s til out of Tag Team.
Q - Unchanged for both forms.
W - If Valor uses the active it will apply Harrier to the next target she hits with a basic attack within 3 seconds. This mark cannot be activated by Valor. Quinn gains +50 basic attack range when attacking targets marked with Harrier.
E - Base damage is increased by 100% and scaling is increased by up to 100/200/300/400/500% based on distance from target. Increasing the closer you are. (Remember her base and scaling on E is not that large to begin with). CD changed to 24/21/18/15/12/9. Procing Harrier damage reduces this CD by 1.5 seconds. Doubled vs champions. (I word damage as R can consume the mark when activating R. Otherwise E->R would have Valor start with a 6s E which will quickly drop with basic attacks. This is meant to be her dive pattern but the 2nd E is cannot be too short to allow counterplay to it's damage.)
More power beyond lanning phase. But a doubled CD on E at rank 1 will really hurt her early game safety. Champions with low CD mobility that she tends to even counter(Riven being the extreme) will now be able to bully her if her E is on CD or she didn't rank it first.
Her assassination pattern mostly requires that her team be there to back her up. If she gets a kill she can escape by going to Quinn form and using E to get out. But E is a risky move and Quinn is squishy. As an assassin she'll still likely get destroyed when going in. It'd like talon going in without his stealth. This is fine as she also gets to be a backup ADC. Talon gets to be a sub-par AD caster without his ulti.
Overall weaker early, stronger mid/late. Reduces what makes her so toxic in top lane. She'd probably move to mid lane as her main lane and situational top.