@Stashu, Cassiopeia in 4.21 and beyond.
So I'll get right to it. She's powerful. Like, probably overpowered. On PBE I could consistently all-in someone at level 2 with ignite and if they don't flash it's First Blood almost every time. Her early, mid and late game poke are crazy and if you land Q-E-E-Q on a target, they'll probably have to go back.
And this all really, really scares me.
Cass is strong, yes, but she isn't anywhere close to what she should be. I'm worried that you'll look at the play rates, win rates, and people saying "yeah she's alright I guess" and decide she only needs a few tweaks, then call it quits. She needs more than a few tweaks, she still needs a massive overhaul on the passive and Miasma, and more power shifted from E to Q.
So lets look at the two big ones then. The passive is still a chore, still unfitting, and even if you make it strong enough and change the bonuses to alter gameplay more than just stat-boosts and heals, it won't be enough. The absolute best version of a Cassiopeia kit would never have something so reduntant and boring as a stacking passive. You tried that idea, it didn't sail well, and now you're trying to patch the holes in it as the whole thing's filling with water. Your job is to make Cass the best she can possibly be, and keeping the current passive will NEVER give us the best version of an evil poison snake mage. You're holding onto this thing for the sake of holding on. People may say they like it more once she's powerful, but that'll all go away when she's nerfed. Where will you be then? Probably changing other cool passives into boring stacking chores cuz we'll totally learn to love it even though it's torturing us, amamrite? What kinda Stockholm Syndrome shiet?
Miasma sucks. It sucks super hard. It's barely used for zoning (as it should be) and almost always used to poison for CS or poison for poke. We already have a spell for that, let's see what other stuff Cass can do. Maybe it corrodes and deals increasing damage to people who stand in it for a long time, maybe the projectile is way slower so it's not as abused in poking but better for planned manipulation of the enemy. Some people may think this is a minor point, but I think Miasma being able to be used as a quick poison takes a lot of value away from the skill it takes to consistently land Q.
You're not even close to done, Stashu. You've addressed the power concerns, and that should hopefully tide us over until we get more substantial changes to the passive and E focused kit we hate. But power issues weren't the concerns most of us had. We want our poison mage back, and what we have isn't a poison mage. Not even close. She still plays the exact same way as before, only with more damage, which could very well be taken away and who even friggin' cares about the bonus damage if it's keeping the champ from being more fun, GEEZ. Like, imagine if Zed's passive instead made his autos do 500 true damage on hit. It'd be crazy strong, but literally every fun part of his kit would have to be nerfed into the ground to compensate. Sacrificing fun gameplay (Q-focused playstyle, managing Deadly Cadance, kiting around a teamfight, etc.) for power (slamming the E key over and over again) is so damn stupid. Why do we have to keep explaining this? It isn't that hard to understand.
Oh, and if you reply to this and only cut out one section of it and ignore all the stuff about the passive (which is like, the biggest, most important thing you need to change about her) LIKE YOU ALWAYS D0... Well c'mon man, it's rude.