How to fix Quinn
The main issue with Quinn is that most of what she can accomplish is done better, specifically by Sivir and Vayne if I had to name a few. After examining her kit I determined that the main two issues were her Passive: Harrier, and her Ultimate: Tag Team/Sky Strike. I would like to preface the following analysis by saying that neither of these skills are, strictly speaking, bad. They just have issues that make them sub-par. I will also be proposing examples of replacement skills to show I'm not just criticizing.
Let's start with Harrier. The main issue is it's random targeting nature and the inability to hold on to it until the ideal moment, both are hallmarks of poor skill when applied to the dangerous role of Marks(wo)man. Because it targets randomly, even when it does favor champions that doesn't mean that it will target the enemy Marksman. In a teamfight you might desperately need that extra damage to take out Yi before he shreds you, but instead it targets Leona because she's two units closer to you. Secondly, if you happen to be running past an enemy minion while you rush to engage in a teamfight or a chase, your Harrier will just pick that minion and waste the 10s cooldown that you might need to initiate the fight or finish the chase.
Replacement: Keep the name, Harrier, just make it useful. Instead of being a random target every 10s, have Quinn gain a buff that makes her next attack result in a follow-up strike from Valor that Marks the target, making the next few (2-4) attacks deal bonus damage (something like 15-115 scaling with level + 0.3-0.4 bonus attack damage).
This would allow her to keep her passive pocketed until she needs it, and removes the random chance of a poor target choice, rendering her passive moot until it re-ups. A Markswoman shouldn't have to worry about her Passive becoming dead weight at crucial moments in a fight. It should be left up to the player to carefully select the correct target and utilize her kit to its fullest. It would reward players who strategically pick out soft targets and follow through with multiple attacks.
Now the Ultimate. This requires a rework entirely. The main issue with it that I see is that, while it does fantastic damage and is excellent for chasing and initiation, it has some faults. The first is that the very concept of the skill, turning into a speedy bird and zooming after a target, often necessitates separating from the team. That runs counter to the survival of the Marks(wo)man role, since unless someone is playing Janna or Twisted Fate, Quinn is probably screwed for backup. As much as it fits her flavor as a Ranger, that should not come at the expense of good strategic play. A ultimate skill should not require a player to abandon his team mates. There's also the issue that Sivir is simply better at the kind of chasing that Tag-Team accomplishes because she brings her entire team with her, allowing for superior positioning plays and greater overall strategic potential. Sure it doesn't have the execute effect of Sky-Strike, but when you're bringing your entire team with you that loss matters very little. The bonus is that it can also extract a team from a bad fight, making it more adaptable too.
Replacement: Once again, keep the name but make it good. Rather than switching places with Valor, have Quinn root herself like Xerath, but then have Valor be summoned and player control switched to Valor. Valor has two abilities: Q - Switch Out: Quinn switches places with Valor, leaping from her rooted place to wherever Valor currently is, and R - Sky-Strike: Quinn fires a massive volley of arrows that impacts after 0.5 second delay. This allows Quinn a huge amount of adaptability, permitting her use of Valor to scout safely without risking herself. Switch-Out allows her to sacrifice damage for superior positioning, while Sky-Strike lets her safely chase down a single enemy or group of enemies from safety. Similar Ultimates are Karthus' Requiem, and Ezreal and Jinx's ultimates. The greater control should mean a lower overall damage output than the other three abilities.
The point of these changes are to make Quinn more adaptable while keeping her theme strong.
