What makes an ADC different from an assassin? [Long]

Let me preface this by saying I like how League is currently and I don't want to change any of the champions as they exist right now. These are just my own thoughts on how the various champion classes are balanced against each other and other ways it can be balanced as new champions are added.

Let's start with how Riot defines the assassin and marksmen classes:

Assassins - Champions able to get in to a protected or vulnerable target, do large amounts of damage in a short window to kill them, then able to get out.

Marksmen - Champions with limited mobility but superior range and damage scalings, allowing them to do the highest overall damage in extended fights.

From these definitions we have a clear distinction: Assassins have short windows of bursts of power they need to play around, and marksmen have long windows of sustained power. Okay, that seems reasonable. But to what extent is this realized? Does the actual gameplay conform to these definitions?

How do Fizz, Zed or Talon match their assassin definition? Well, their kits have more mobility in them and their damage is on their abilities mostly. So in practice they can make more picks using their mobility and their damage is gated by their cooldowns and mana/energy. The "short window" part is fulfilled as well as the get-in/get-out part. That more or less fits the definition of an assassin.

What about marksmen like Cait, Draven or Jhin? Well they're ranged for one and have low mobility since most of their spells are damage focused. They also have the highest burst and sustained damage in the game after 1-3 items due to their superior scalings. Okay. Long range? Check. Low mobility? Check. Sustained damage? Check. Highest burst? Hmm..

But I thought assassins should have the edge in short windows?

Wait, how can you have superior sustained damage but not also the best burst damage? ..

^^ This is the question I think needs to be asked whenever a marksman is designed. Jhin in a way fulfills this since his attack speed is gated. He has great damage but it's inherently spread out over time through this attack speed limitation, hurting his burst but not affecting his overall dps.

There's actually multiple ways to achieve this trait of lower burst but higher sustained dps. Another that comes to mind is a champion that utilizes accuracy as a stat. Meaning if they have 70% accuracy but 50% better damage overall (meaning 120% effective damage), they would be doing large amounts of sustained damage over the course of many auto attacks but be quite unreliable in duels vs. assassins and fighters who's damage is guaranteed and consistent. The crit mechanic already does this to a lesser extent, this would just be a further expression of it.

Another way to further differentiate the two classes, going with the theme that assassins tend to be melee, is to nerf a marksman's damage at shorter ranges. There's many ways you could do this. As inspired from another member on these boards, you could have a marksman with a long rifle that can't shoot when in melee range but instead starts hitting enemies with the butt of the gun with a slow attack animation. Or you could have a champion that can't attack at short range at all but has small movement/displacement spells that allow them to increase the range when needed.

Overall though I think League strikes a nice balance. These are just ideas to serve as inspiration for future champions to more fully embody their roles and make each class more defined, varied and interesting to play as an against. Thanks for reading!