Seraph's Hymn at level 1 sounds good, it's actually a decent option especially if you have other teamates who can blind people, however Charged Strikes is usually better since it's more consistent and makes your AA splash.

Ring of the Leech is the go-to for level 4, Inner Light is not really a good idea, and Plate of the Whale is more of a late-game talent and it's pretty situational.

War Traveler is the only option at level 7, since it increases your mobility. Impale is not needed, and Surge of Light bring a bit more burst at the cost of safety and consistency.

Ball Lightning is not a skillshot, and it usually gets good value no matter what. Valkyrie has its uses and can offer great opportunities, it's really your choice here.

Both Grounding Bolt and Thungergod's Vigor are great at level 13, it depends on what you need especially in Q build. Lunging Strike is not a good idea, more on this in the last build.

Pierce is very strong especially in Q build, Marital Law in AA build, and Penetrate is once more not something you want.

Infinite Lightning is very strong in lower leagues, or if your team has some way to keep the enemies close to each other. Imprisoning Light is a game changer but not consistent, and Titan's Revenge is the consistent option and it's very strong in AA build.

You want to use this build against high mobility targets such as Tracer or Lucio, but it's always good in general.

Charged Strikes will give you AOE damage on heroes and you can hit heroes that are not in range by hitting a closeby non-structure unit.
Ring of the Leech is your sustain, and it sinergizes with the bouncing AA from before.
War Traveler increases your overall mobility, it's the best talent to be able to play safely and do your damage.
Ball Lightning is a great ultimate in teamfights, it's not a skillshot so you can finish someone with it pretty easily.
Grounding Bolt is your CC tool, the slow it provides will help you chase people, even more since you have your own movement speed increased.
Martial Law synergizes with your previous talent and whenever your team provides CC. It also works very well with the splash AA from level 1.
Titan's Revenge will finish this build perfectly, as it not only makes your AA ignore Armor (RIP Garrosh), but also increases its range!

This is also a very versatile and powerful build, however you need to hit your Q spell consistently on your foes!

Thunderstroke tells it all, you want to hit Q and increase your damage!
Ring of the Leech is still the best option for burst healing in case you're dived on, Plate of the Whale is the other one since you're very reliant on poking and staying at range.
War Traveler increases your mobility, meaning you can get in range for Q and get out safely.
Ball Lightning will do the job in teamfights.
Thundergod's Vigor will reduce the cooldown of your Q spell, however the slow provided by Grounding Bolt can really do the job.
Pierce is your only option, AOE slow and damage or reset on Q spell!
Infinite Lightning is a great powerspike in the chaos of a teamfight.

This is something you will see a lot and is pretty popular, however it's not a good idea and that's the main reason why people say Cassia is weak against mages.

Basically people use E to go in and deal their damage, but while you're using E, you're not moving anymore making you an easy target for things like Jaina. It's what I call the Cassia-Greymane Syndrome, people want to go in to do the damage, however it's better to poke with cousistent damage and wait until people are low to go in.

Fend is an ability you really need to be cautious using, and as such building on it is very risky against opponents who know how to focus a target.
Impale at level 7 increases the damage on lower health targets, this is actually viable, however it is absolutely not worth trading the mobility offered by War Traveler.
Lunging Strike will increase the area, but you will be immobile for 0.5 more seconds (non-neglectable).
Penetrate sounds good, but it requires to have someone stay in the area of damage so either a noob or a disabled target, the later being destroyed by Martial Law's percentage damage!