Sorry for the long post, but I really think that Riot has missed the mark on this champion update. She just isn't the same champion anymore, but there is hope, because some of her changes show a lot of promise.

I'll start off by stating what defined Cassiopeia

She had the opportunity to regulate her safety to her damage output by managing how far away she was from champions.

1. Stay outside Q range and weave in range to do some minor poke damage. Her large range let her do this very safely before.
2. Stay inside Q range and casting the occasional twin fangs when it is safe to do so.
3. Full on nuke within retaliation range of most champions with twin fangs spam.

I can not state enough how important this was to her play style. At the moment her terrible Q range, and movement speed changes has completely gutted this. Notice how the miasma and twin fangs change has no part in this. Those should be balanced without interfering with these two core aspects of Cass.

The passive: In all honesty I don't think this passive has been thought through well enough. When the average game lasts around ~30 minutes this passive does more harm then good and in my opinion should be reworked completely. Being slower than your opponent makes it impossible to land your Q's. Lets do a quick comparison to Karthus (as he has basically the same Q). He is able to catch people at a large range with his wall (and slow them for a few seconds), and then follow up with his Q. Cass has miasma but by the time you you actually get in range to follow up with your Q they aren't slowed anymore. This change combined with the Q range nerf has made it practically impossible to initiate on an opponent who is expecting it or running away. I don't think reverting the Q range is enough on its own either. There is no cheap option for movement speed early game now that we can't buy boots, so we can't counterbuild against faster opponents. 15-25 MS from Ether Wisp isn't enough, and we have other stat priorities with the current changes (read further down).

Twin fangs: I personally don't like it, but I don't think this spell NEEDS to be changed. It just forces you to get some early cooldown reduction, on top of some mana before you can start doing large amounts of damage.

Suggestions to keep her feel:

1. Revert the Q range change, if necessary change the movement speed increase as well. If this isn't done Cass will have lost her old playstyle. End of story.
2. Rework the passive somehow, it is a MAJOR problem for cass to be slower than her opponents. Having a lower movement speed than our opponents indirectly shortens our Q range because of the 0.4 second delay.

If this is done the major differences between 6.8 and 6.9 will be:

1. Unique opportunity to stop dashes/flashes + Better miasma
2. Farming with twin fangs
3. Nice lifesteal synergy with E poison + new Catalyst

Negatives:

1. Needs more time to rampup due to the new need for CDR - Offset a tiny bit by twin fangs farming
2. No opportunity to self cast miasma

The rest of the balancing is what you do best: Changing ratios, base damages, and cooldowns.

Again, sorry for the wall of texts, but at the moment I have quit playing League because of this change (I've been playing cass ~70% of my games since S2)