I have huge concerns about a ranged AD assassin at all, because I think stacking AD on a champ that can auto attack from a distance will be precarious from a balance standpoint. So if Riot was going to turn her into an assassin, what do you think they could do to keep her from being unfair? Because right now, that would involve using ult, then using vault from a distance to proc ult, vault, and then auto a couple of times to get huge burst out, and after that she has ranged autos to follow up.
Ideally, if Riot ever decided to make Quinn an Assassin then I think decreasing her range would be the natural first step towards making her fairer to play against, while also freeing up some power budget in her kit to allow for more Assassin-like qualities. This means either decreasing it by just a few units (to about 400-500 range or so) or flat out making her melee.
Yes, there is the argument that removing Quinn's range would destroy her identity, but in my (humble) view, I don't agree that it's essential to her character, and think she could make a lot of sense as a melee also, both thematically and gameplay-wise. Of course, it would be somewhat of an ambitious change, but Riot has proven with all their past reworks that nothing is ever too sacred to be removed from League if there's a better and more satisfying option (just look at how different Graves is now after his rework, or Sion, or Galio).
If decreasing her range isn't enough, I'd have her Vault modified to work like a skillshot, so that she can dash in any direction she pleases, but stops at the first enemy champion or large monster she collides with before knocking them up and dashing back. That'd make Quinn a lot more mechanically demanding and less binary to play against than she currently is, I feel.
Also, Assassins don't generally make use of attack speed, so the attack speed steroid on her W passive could go or be replaced by something else if appropriate.
That pattern, imo, would have to change for her to be a healthy assassin. Do you see a way to do that without killing her identity?
It all boils down to what's truly core to Quinn's gameplay, and what we consider her in-game "identity" to be. There's bound to be disagreements on that front, but I'll tell you my thoughts on what makes Quinn, Quinn:
Core Gameplay Trinity
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High mobility (good chase and escape potential; at least moderate roaming potential)
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Physical/AD burst damage (should be somewhere between Skirmisher and Assassin levels of burst)
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Vision utility (anything from AoE reveals to single-target truesight)
Core Thematic Trinity
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Falconer (works in tandem with Valor for most if not all of her abilities; feels like playing both a human and bird of prey simultaneously)
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Ranger (can span large distances or close gaps quickly; prefers solitary ambushes to large-scale teamfighting; is independent and can hold her own against enemies)
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Scout (possesses modes of reconnaissance such as vision reveals and camouflage)
Everything else, in my view, is on the line in terms of a full-scale rework, because it isn't really core to Quinn in the long run. I love the knockback on her E, the omnipresence that her ult brings, the nearsight on her Q. Ultimately though I think I'd trade all those things for a more effective, well-rounded and thematically cohesive version of the champion.