Personally I have little to no hope for the Assassin Roster Update

Because if we have learned one thing from older updates it's that when a champ needs a LOT more individual attention than just maybe one new ability and a few tweaks and maybe a gimmic, the update is bound to fail - see Mordekaiser or Skarner for reference. Some champions can get easily away with it, but especially in the assassin group there are many that are really, really, really old. Most of them were already in the game when i started playing, and I tell ya, that was some LONG time ago.

The most recent assassins are Kha'Zix, Rengar, Ekko and Zed, and I think whatever problems they have could be fixed with some tweaking. Diana might also fall into that category. But those are not the ones that worry me (although I'm curious how you want to make Rengar "less unfair" when his entire thing is "jump on squishy and tear them apart").

I'm more talking about the classical assassins here:

Most of those champions are in dire need of an overhaul, both in the gameplay, the narrative, and the visuals. I'm not saying they are weak - sometimes quite the opposite -, but their design is clearly problematic. I mean, Kassadin for example has been Riot's overperforming problem child since the start of season two and the only way to even make him stop being banned was pretty much breaking him apart in early/midgame. Or look at Fizz: The reliability of his damage as well as the invunerability and versatility of trickster will make sure that he is never truly a bad pick, unless the enemy team hardcounters him directly, and even then he can still be dangerous. But that's is the core to his design. No matter how you try to shove and move things around that, the core problems remain.

Also, most of these champions lack any narrative or gameplay fantasy. They are archetypes at best, and their unique gameplay fantasy is not exactly unique: it's "I want THAT fucker dead. So I jump on him and make sure he's dead". That's the GENERAL assassin fantasy, but nothing that would be fit to describing an individual champion - and the lack of narrative is playing into that. Sadly the Riot Lore and Foundations teams have the content release speed of a snail on valium. The reboot is still not done, some champions have no reason to why they actually exist in this world, etc etc. The fact that we have had few Shurima storys don't really change that.

Fizz is "this fish who's people vanished". Akali is "that ninja girl - a walking stereotype". LeBlanc is the "evil mastermind", but that's more an archetype than a character. Shaco and Eve are just "know one knows. Literally. Not even the lore team knows what these two are." That's not really something you could built on from a design perspective when it comes to making that characters feel like they should in the game.

A roster upgrade would be too little ressources for most of these champions, and it would end up "Mordekaisering" them: either they have them be balantly OP for their gimmics to work somehow, or they are left to rott in the Skarner-corner, where they enjoy a lot of free time because no one ever bothers to play them.

So unless you massively increase the scope of this roster-upgrade to a Mass-VGU-Event, I have little hope for any of this to work out even remotely fine.