Camille could have been a great assassin.

In the current meta they're effectively crucial. Having an extremely fed Vayne or Twitch on the enemy team with no means to kill them is where Assassins come in.

Extremely fed Vayne or Twitch is exactly where you prefer a diver who can not only deal damage, but lock them down for a while. Zed or Talon are likely to get killed by them when they try to gib them, unless they are also extremely fed. As I said, assassins have no use-case. They are in the game simply because some people like them.

As much as it would be a bad move to introduce a new Assassin at the current time, it's never stopped Riot in the past.

No, not right now. Riot haven't released any assassins for more than half of this games lifespan.

the core problem with assassins as a class is that right now, at least in the minds of the community, what defines a well designed assassin character is whether the enemy champion is dead at the end of the combo.

When riot figures out how to give assassins a better core game play fantasy than "you = dead", than we can have more of them in the game.

by better i mean you get to have the game play fantasy even in a condition where you have to run without achieving anything in the game and lose objectives.