the only passive that I can think of that incentives you to stay longer in voidform is Mass Hysteria. Mass Hysteria increased the damage of your DoTs for every stack of voidform and dropped after you get out of voidform.
here is where it gets confusing.
Blizz gave us Voidform because they “felt” that shadow played to much like Aff warlocks, however voidform mechanic when introduced was basically a less efficient Aff Lock.
You ramp up you DoTs the longer you are in voidfrom. Aff Locks whole playstyle is around managing your DoTs and ramping them up. its just shadow is less efficient because you don’t actually have direct control over the ramp up.
you see how from the get go the whole spec and Voidofrm had an identity crisis.
in BFA they tried to push it into another direction but it obviously didn’t work.
I think ramp up in its entirety needs to go.
there is too much going on with the set up of the spec. you have to first build up insanity to enter voidform then in concept you would want to try and keep up with the drain until you have enough stacks to do good dps. they need to choose one of the two systems.
either have it so we build up insanity to enter vodiform and once you are in vodiform you get an upfront fixed buff and drain or…
have it so it’s a heat build up system. Meaning rather then building insanity to enter voidform there is a drain that push’s against your insanity generation and the more your insanity bar is full the more damage you do. think of it as you are always in voidform but rather then getting kicked out once your insanity hits 0 it just remains there and you can just keep generating it as it depletes.
you could attach a tier system to it. So every 20 insanity is a tier adding up to 5 tiers. Each tier provides a buff, for example each tier provides %3 increased haste and damage adding up to %15 haste since they want the spec to be oriented around haste. they could add a visual indicator of each tier.
all you have to do to get the buff of the tier is to keep your insanity bar in the range of that tier. For example, you want to stay in tier 5 you just have to keep your insanity bar above 80, tier 4 is between 60-80, tier 3 is between 40-60, tier 2 is between 20-40, tier 1 is between 1-20.
by the way the heat mechanic(tier system) is not a new idea Yvaelle proposed it I believe way back in BFA alpha.