Her current passive is this since Patch 6.9:

Serpentine Grace: Innate: Cassiopeia gains 4 − 72 (based on level) bonus movement speed, but she cannot purchase Boots.

NGL it’s a shit passive. The ever increasing MS is really strong on it’s own righ the ability to purchase 6 items is handy, like in 2 out of 10 games. Having the ability to skip boots allows you to get items a bit faster but honestly 1100 gold for any boots is a great asset at the early stages of the game.

Cassiopeia players know how much they miss with this. Players only understand the importance of boots when tehy no longer having it.

Even the ability to choose one of the boots passive would be a huge buff for the champion but I’m asking for something else.

I honestly believe that today, one of her older passives could make a return

Aspect of the Serpent (V1)

  • New Innate: Cassiopeia permanently gains a stack of Aspect of the Serpent for:
    • Poisoning an enemy champion (per second)
    • Killing a poisoned unit
    • Casting Twin Fang on an enemy champion.
  • A maximum of 400 stacks can be held, and grants different bonuses based on the current amount:
    • 75 Stacks: 5% increased ability power. Twin Fang now heals Cassiopeia for 6 / 8 / 10 / 12 / 16 (+ 10% AP)
    • 200 Stacks: 10% increased ability power and gains 25% cooldown reduction.
    • 400 Stacks: 30% increased ability power.

It was later changed to aother version in 4.21:

Aspect of the Serpent (V2)

  • Now passively generates 1 stack every 6 seconds.
  • No longer generates stacks from unit kills.
  • No longer generates stacks from using Twin Fang against champions.
  • Bonuses benchmarks increased to 100 / 250 / 500 stacks from 75 / 200 / 400.
    • On average, the bonuses are unlocked more quickly

V2 is pretty dumb but V1 has potential to make a return with adjustment to stackings and rewards. Players have long been complaining that Cassio used to have more relevant heal thats still far from what we experienced in S11-S12 before changes to healing. This self healing battlemage thats also far from Vlad is still amazing concept.

This passive granted her extreme amount of late game power, but honestly she is already a good scaler and late game is less and less an issue the higher elo you are getting to.

Obviously this passive should be restructured, by increasing the amount of stacks you nee to collect and tune the abilities it provides toward a new direction. It was removed in a time where it wasn’t really fitting for any champion but lately they made Viktor, Syndra and Aurelion Sol stackers and they work pretty well honestly so I don’t see any reason why this should not return for her.

I seen some interesting (but semi broken) concepts on reddit and discord and honestly if scaling, numbers are getting readjusted to make it somewhat balanced I can see these changes happening because tehy are making sense.

Aspect of the Serpent:

  • Minions and Small Monsters killed by Noxious Blast or Twin Fang give 1 stack
  • Siege Minions, Super Minions and Epic Monsters killed by Noxious Blast or Twin Fang grant 5 stacks
  • Champion takedowns grants 20 stacks
  • 200 Stacks: 5% increased ability power. Twin Fang healing increased by 20% of tAP. Cassiopeia gains 1-18 Magic Penetration based on level (1-18).
  • 300 Stacks: 5% increased ability power. Noxious Blast aplly Griewous Wound. Cassiopeia gains 1,6% Tenacity based on level (1-18)
  • 400 Stacks: 5% increased ability power. Cassiopeia gains 20% Ability Haste

This is just one version that feels really broken but I can see a balanced version of the stacking mechanism returns one day to help out her neglected kit.

At the moment the champion’s pick rate is constantly falling with every patch, most for meta IK, but that’s a trend which was true even when she was meta albeit in smaller reduction rate.