Since the BfA prepatch, Shadow has been suffering from a case of cognitive dissonance: voidform works thematically as well as mechanically if your goal is to stay in voidform as long as possible. Currently however, it is more beneficial to aim for casting as many void eruptions as possible which is counter-intuitive to prolonging voidform. The issue is further complicated by the fact that simply nerfing void eruption while buffing voidform will reintroduce the same problem from Legion where Shadow had a severe ramp up time.
In order to find the happy middle ground we need to:
1) Nerf void eruption damage to make staying in voidform as long as possible the better DPS choice.
2) Give voidform a unique identity by adding the following effects:
a) LotV baseline.
b) Cast while moving effect from StM baseline while in voidform.
c) Vampiric Embrace now a baseline passive of voidform (VE might require a nerf for this to be balanced).
3) Make the following talent changes to work with the voidform changes:
a) San'layn just increases self-healing while in voidform. CDR effect removed due to Vampiric Embrace being a passive of voidform.
b) StM now just increases insanity generated by 100%. Now a level 45 talent in place of Dark Void.
c) Dark Void could now be an AoE burst talent in place of StM: rather than applying SW:P to all enemies surrounding the target, it now consumes the rest of the time on all your DoTs, dealing their damage instantly (similar to an AoE Deathbolt that AffLocks just got in this patch).
d) Since LotV is now a baseline passive, that talent slot is now for shadow priests who hate multidotting: Mind Blast (or SW:V) now applies any DoTs on the target to all nearby enemies.
e) Dark Ascension now activates a voidform with full insanity, cannot be used while already in voidform, and returns you to the same amount of insanity you had before you used it (allowing you to potentially chain 2 voidform phases).
4) Change Mind Sear back to a passive of Mind Flay that damages all enemies that have SW:P on them allowing for spread AoE.
I believe that these changes should:
A) Give voidform a reason to exist while maintaining its unique mechanics but not making you feel useless outside of it.
B) Decrease the ramp-up time baseline allowing you to have the mobility and self healing of voidform more readily available while leaving the option to stack insanity to 100 for greater reward if you dont need the mobility/healing right away.
C) Since we usually need to multidot before using Mind Sear anyway, decrease ability bloat and allow for spread AoE by rolling Mind Sear back into Mind Flay.
D) Provide 3 unique lvl 100 talents to either remove the need to multi-dot (replacement talent for LotV), chain voidforms (new Dark Ascension), or AoE burst (new Dark Void in place of StM).
I don't believe I missed anything, but if I wasn't clear on something please don't hesitate to ask for clarification. As always, all feedback is welcome