Adding an aiming animation to ranged auto attacks would do a lot for champion diversity/viability
##Foreword I'll be addressing many concerns in this and it's likely yours will be included, so please read the full post before responding!
#WHY? Why introduce an aiming animation to ranged autos?
Range is an inherent advantage against any melee champion. A player that knows how to properly kite will always dominate melees until every melee in the game is given insane mobility/CC spells to compensate (aka mobility/CC creep). This hurts champion kit diversity and viability, especially tanks/juggernauts where high amounts of mobility in their kits don't make sense thematically or gameplay-wise. It also hurts lane diversity, making it rare and difficult to make a melee viable as an ADC and not be completely broken.
Aiming for ranged champions also just makes intuitive sense. If you try to hit something from far away you have to aim first. You don't have to aim if something is directly in front of you. An aiming mechanism would provide an inherent point of balance between the eternal melee vs ranged matchup.
#WHAT? What is an "aiming animation" and how is it different from a skillshot?
A skillshot is just a projectile that you give a direction to that has no guarantee of hitting a target once it is executed. An aimed auto attack is when an aiming animation occurs before an attack animation and is guaranteed to hit the target if the aiming and attack animations aren't interrupted.
#HOW? How might a mechanic like this be implemented?
Not much actually has to be changed. A target could simply be right-clicked once and a scope appear under the selected target (seen only by the marksman). An advancing arrow immediately moves outward from the marksman's character model toward the scoped target (symbolizing the process of aiming). When the arrow reaches the target, the attack animation would begin immediately. Marksman should be able to move while aiming so it's not a disruptive animation like the attack animation which requires them to be stopped with no other commands issued. This means they can still kite, just that the time between autos is increased based on their distance from the target. Every time they switch to a new target or complete an auto attack, a new aim must be taken.
#Additional Thoughts/Concerns
How does this weaken marksmen other than slowing down their attacks from a distance?
A subtle "nerf" is introduced to marksmen attacking from further away. Bushes, stealth and invisibility (tools mostly used by assassins) are now much more powerful against marksmen attacking from afar since enemies have a larger window to go out of sight due to the increased aiming animation, thereby cancelling the auto.
If aiming time increases with distance then wouldn't marksmen be stronger up close and weaker further away?
No, because their base stats will be worse than a melee's. This means that, all things being equal, up close a melee has better stats and should win even though a marksman is doing their highest auto attack dps. However, if there's little threat of them dying then it might be a very good decision to get within melee range and maximize their dps.
If marksmen are worse up close and have their lowest auto dps from max range, what are they good for?
This is where a philosophical difference in design comes in. It's not that marksmen are strong from afar, it's that they get to attack from afar. Why is this point important? Because it opens up so many more kits to be viable. By creating an inherent weakness in ranged autos (an aiming tax), now melee kits can be viable that aren't simply CC bots or have some insane mobility skill taking up a slot that could be used for something else entirely. Melee ADC/APCs could become viable. The entire kit of a marksman would have to be taken into account to know what their weaknesses/strengths are. A marksman no longer needs to be defined by the fact that they attack from a distance, instead it's just another aspect of their kit.
