Edit: WORK IN PROGRESS - adding to post as more ideas come up or people suggest improvements.
The devs recently mentioned they were going to look at Shadowpriests first on the list of class specific updates for Battle For Azeroth.
They have also mentioned they were not planning on large overhauls this expansion to most classes, but since Shadow was called out as top of the list - I think we need to be the most prepared and most vocal upfront to get our feedback in when they want to receive it.
To that end, I'm dredging up my past Willpower/Insanity design from last expansions alpha, incorporating all the best things I like about Legion design, and slapping on anything new I think we need as well:
Two resources: Willpower and Insanity.
Every tick of a Damage Over Time (DoT) provides 1 Willpower. 1 Willpower = 1% haste. Max 100 willpower.
Every use of an Empowered spell (see below) inflicts 50 Insanity. Insanity decays 5 per second. Max 100 insanity.
What happens when you hit 100 insanity?
Apotheosis happens. The spriest loses control of the old god magic they purport to control through their sheer force of will(power): this is both good and bad.
- removes all DoTs from all targets
- removes all Willpower (and corresponding Haste)
- reduces the Spriests movement speed by 50%
- doubles the rate of insanity decay (from 5 to 10)
- causes the Spriest to lose 10% max hp per second
- all spells cast during Apotheosis are Empowered (see below)
- Empowered spells do not inflict Insanity
- can be /cancelaura'd early
What are Empowered spells?
Empowered spells have increased effects but inflict Insanity upon the casting Shadowpriest. Each use of an Empowered spell inflicts 50 Insanity.
Desperation - no cooldown, does not trigger the GCD - upon using this spell the icons on core spells will change, becoming Empowered By The Void: the next core spell used will gain increased effects.
Empowered Shadow Word: Pain becomes Devouring Despair - a powerful DoT that ticks every second for 10 seconds and heals the Spriest for 3% max HP per tick
Empowered Mind Flay becomes Void Torrent - deals massively increased damage and lasts 5 seconds
Empowered Mind Sear becomes Shadow Crash - deals burst aoe damage after a delayed travel time
Empowered Shadow Word: Death becomes Forbidden Word: Null - no longer requires a low health target, always critically strikes and damage scales with critical strike rating. Deals 50% less damage for each of the Spriests DoTs active on the target.
Empowered Fade becomes Disapparate - reduces threat, teleports the Spriest 30 yards in the direction they are facing, and causes the Spriest to be immune to all damage for 1 second
Empowered Silence becomes Excommunicate - locks the target in horror instead of silence and knocks them back 30 yards
Empowered Dispel Magic becomes Nullify Magic - dispels all harmful magic from the target ally and the Spriest
Remove the following spells:
- Shadow Mend
- Power Word: Shield
- Shackle Undead
- Mass Dispel
Change the following spells:
- Vampiric Embrace heals for 10 seconds every 2 minutes
- Shadow Word: Death has one charge (down from two), but hits harder as a result
Thoughts
- empowered spells are a way to give us a lot of our favorite spells back (Devouring Plague, Psychic Horror, etc) without bloating the spec or increasing complexity
- empowered spells add a lot of situational power to the class, but mobility and utility cost damage
- removing utility spells (Mend, Shield, Shackle) reduces the potential maximum utility the spec can offer, which was probably hard to balance
- the spec has less variety of utility (and less spammable), but powerful utility in specific niches (Nullify Magic, more frequent Vampiric Embraces)
- the spec has few spells, but needs to make complex choices about which to empower and when
- the insanity mechanic is an amplified Floating Energy Pool system; capping out is potentially fatal under the wrong circumstances (homage to Surrender 2 Madness)
- Disapparate can use the Diagonal Slide graphic that Archbishop Benedictus has during the Cataclysm quest series (video here)
Gameplay
Due to the extreme power of haste toward sustained fights and especcially the benefit overcapped (more than required to hit the GCD) haste provides to DoT specs, maintaining maximum Willpower is always more valuable in the longterm for DPS than triggering Apotheosis. However with seconds left in the fight, or when pulling a small pack, a Spriest could intentionally trigger Apotheosis (removing their dots and accumulated haste) to deal a short burst of damage (Mythic+ trash burst, world questing, ending every raid fight with a bang).
There is an Empowered spell for every different fight scenario, and they are frequent enough to make in-combat decisions about which to use and when - but it takes Insanity management to plan ahead.
For Council fights (Demonic Inquisition), Devouring Despair (Devouring Plague) can potentially hit multiple targets, so you get an extra DoT to manage.
For Single Target fights (Goroth), Void Torrent is ideal.
For big trash pulls (every Mythic+ ever), Shadow Crash can do work.
For bursting something quickly (a jerk world mob that knocked you off your mount, or the fire orbs on Botanist) Forbidden Word: Null (homage to Mind Spike, but good this time) is an curt rebuttal.
This resolves the ramp-up to Void Form problem because Void Form (Apotheosis) is always 2 empowered spells away - even fresh into combat - and since the benefit is Empowered spells, Shadowpriests essentially have no ramp-up at all on short fights: problem solved.
However, for longer fights, and particularly ones where DoTs are an interesting/desirable mechanic - ramp-up actually feels good and is part of why Spriests can feel powerful/fun (the pay-off for ramp-up is higher long-term damage, theoretically). To that end, and virtually all raid bosses, we would not want to trigger Apotheosis at all - our goal would be to maximize Willpower (= Haste) quickly and maintain it through the entire fight, using Empowered Spells only sparingly to avoid hitting Apotheosis: gaining temporary burst damage/utility, but losing our Willpower/Haste.