I don’t believe this is necessarily as difficult as you make it sound. It has always been in WoW that some classes require a lot more effort to do decent damage than others. A talent that increases your spec difficulty and makes it so that it is harder for you to do damage is not an inherently bad thing if there are people who want that challenge or enjoy that playstyle. WoW isn’t just about “how can I do the most damage with minimal effort.” It is okay for a Legion Shadow Priest to perform at 100% only to do equal or slightly more damage than a much easier class.
It is less okay for that to be forced onto the people. What I am suggesting has minimal cost - Imagine on Row 8, you have a column of talents splitting off. The only other change that needs to be done is move devouring plague in a spot that makes it not completely mandatory in the path.
- Madness of the Old Gods: Your void form grants stacking haste for its duration, but drains increasingly fast. (You can also put a “But X, Y, and Z spells do % reduced damage” to make it so that people who don’t like the playstyle don’t feel pressured into it)
- Lingering Insanity: Haste from your Voidform persists after it ends for X seconds.
- Surrender to Madness: All your insanity generating abilities generate 100% more insanity, and you can cast while moving until you exit voidform. Then you die. Horribly.
This still leaves room for shadow priest to be something else - it is entirely opt in.