Shadow priest has some cool aspects, but it lacks some cool factor
Void eruption is really cool, but its only one hero spec once 2 min and the build isnt as good for lower end and pug content most people do as it focuses more on big packs, maybe make it baseline and make a choice of cooldown and duration or something.
It lacks nice feeling nuke ability, mind blast looks boring and improved version in voidweaver isnt better, give it cooler animation
multi dotting feels like a chore and if you mess up shadow crash its a pita, having empowered shadow crash that scales size/targets depending on time with more impactful animation would help, you could just remove shadow crash and make vampiric touch be empowered or replace it with some other function or merge with void torrent dropping a crash every x seconds and refreshing dots. alternatively vampiric could be made instant cast, so you can dot while moving, but shadow crash could be empowered way to spread dots on larger aoe pulls
full shadow look could be replaced with something that allows to see transmog with hints of void, like void wings or something.
SW:Death could be improved, maybe better animation and instead of having to press it multiple times it could recast itself if procs