The current state of Cassiopeia and how to fix her.
Cassiopeia hasn't been doing so well since receiving those nerfs back in 7.18. Riot was trying to steer her focus more towards lategame while cutting down her early to midgame power, however that made her pretty much unviable in a meta-game that seemed to get shorter and shorter with each passing season. The average game lasts 25 minutes now with most games already being decided after 15 minutes, that's the kind of meta we're dealing with.
I'm aware of the buffs on the PBE for her (increased E-scaling), however they're not the solution to her problems. She'll be stuck getting 3 mana-items before building any actual damage and games are already decided after that. Right now, Cassie is sitting at a 45% winrate on diamond level and above, and she's undescribably bad below that. At the same time there are 20 other champions who are within the range of a 55% winrate and don't seem to get any nerfs at all which isn't very nice.
There are some ideas I'd like to bring up on how to approach this snake problem. First of all, an update to her passive has become absolutely mandatory. It's unacceptable that she's omitted from getting flat mpen while every other mage in the game can deal true damage to their lane opponent from getting a simple pair of boots. Meanwhile that 6th item-slot of hers serves no purpose as long as she'll never reach that full build state. It feels like an extreme handicap and not a strength, I'd gladly trade it in for something else.
The least Cass deserves would be a secondary passive that gives her one point of mpen for every level she acquires (thus arriving at 18 mpen like everyone else). She'd also need her basedamage nerfs reverted since she won't be making any use from increased E-scaling due to being forced to rush 3 mana-items just to function, this leaves her with very little AP to begin with.
Now if you truly care about her balancing, you should strive for a final rework. However this time get rid of her Twin Fang ability, it's literally the source of ALL her balancing issues. Having a low cd point and click ability as your main source of damage is a terrible design, she'd be far better off by relying on skillshots entirely as long as she gets some of her power back.
Turning her into a proper poison mage would easily be the most healthy approach if you're looking for a long-term solution.