I think a mechanic more similar to thermal void ice mage on live now would be beneficial; Instead of having damage ramp up over time in void form, just let it give a flat bonus instead. We would be fighting to keep void form up as long as possible to increase the overall up time of the buff.
This way, damage isn't loaded in the back end of void form, and void form won't scale out of control if someone somehow manages to keep it on for 30s+
For now, I'm setting my mind on druid; 2-3 dots + built in baseline dot + good aoe + decent single target + instant cast spenders + heals; seems like everything a shadow priest aims to be honestly. What i'd miss though is the shadow priest's old god theme and "body and soul".