I have only been playing evoker for about a month but from what I have seen and heard, Firestorm has never been good.
Why?
What would it take for Firestorm to be good? How would it need to be changed for it to be as good or better than current AOE setups?
The biggest problems with firestorm are twofold:
There’s no good path to take the talent. You lose an immense amount of single target for very little AOE gain, which brings me to the second point.
The spell being a long cd and cast time with meh damage is bad enough, but both of those things in a ground targeted ability when things rarely stay in it makes it a non-starter, as you can rarely even get the full AOE value from it.
To fix firestorm, blizzard needs to firstly make the pathing in the talent tree less punishing to our ability to do single target.
Additionally, they need to tweak the ability to behave similarly to ravager (will follow the target) if they insist on keeping the cast time and cd what they are, or make it a quicker/instant cast with a much lower/no cd if they won’t make it follow a target.
The evoker tree in general is pretty bad. They didn’t want to design good actives and flesh out the kit with procs or modifying interactions, so they filled it up with too many passives.
Thr end result is we have our core kit dependent on all the throughput passive talents, so there is no room to pick for playstyle instead, which is what talent trees should be.
The other problem is they never bothered to tune it appropriately. The last time they looked at it, they put in a choice node for it that half addresses what the spell needs but then slaps an opportunity cost, so the improvement does not go far enough.
A simple change like a proc from fire breath ticks or pyre casts resetting the cd and making it instant cast would go a long way.