I have a few suggestion for the shadow tree, leading to the lower third and to capstones:
-Remove Ancient Madness, move Spychic Link to its position and increase the links to it.
-Create four distinct roots, one for each damage profile and have one Idol for each damage profile (basically mostly changing the Idol’s of C’thun and N’zoth):
- Pet:
Mindbender talents… leading to Idol of Y’shar
2)Dots:
Mind sear (enable SotV for aoe), Screams of the Void etc.… leading to Idol of N’zoth (swapping the Idols of N’zoth and C’thun from the current built) and enabling the stacks to detonate on mob death.
Unseen whispers: “Learn Void Eruption for even more benefits with mastery!“
3)Aparitions:
Shadowy Aparitions, Auspicious spirits (maybe less + %dmg) AND Tormented Spirits, etc. … leading to Idol of Yogg-Saron
4)Direkt damage and Mindflay:
Mindspike etc., utilizing Psychic Link and Deathspeaker for aoe …leading to Idol of C’thun;
change the Idol’s proc to direct damage spells and mind flay (possibly enable proc from single target damage share via Psychic Link and Deathspeaker for aoe)
Sounds invading your mind: “Embrace the Dark Ascension for even more damage!“
alternatively keep the postions of C’thun and N’zoth where they are in the current builds tree and change Idol of N’Zoth to work with direct damage and Mindflay (although I think N’Zoth is the better Capstone for Dots)
-keep the current possibility to reach two capstones, allowing the players to build their own Shadowpriest by matching up to two dmg profiles:
You want to lean towards direct damage spells to get the maximum out of your mindbender? Go 1+4.
Want to wither your enemys away with dots and shadows? Go 2+3
You seek to become the ultimate zoo keeper of shadows even you can barely controll or not at all 
? Go 1+3
These and more fun combinations of ways to unravel your enemies and your own mind.
Edit: No Idea why 1) is kept formated into 1. and closer to the middle