Just wanted to add a collection of ideas to see if the Devs liked any of them. Its a major wishlist/brainstorm so not everything will be balanced.
Vampiric Touch - Changed to Ice DoT and a Snare(glyph) for purposes of being an opener in pvp and to free up mindflay itemization.
Vampiric Embrace - Changed to a 30min raid buff that effects only 1 target. Healing will be siphoned to the target based on the dmg done by the Shadowpriest. Limit one VE per person in case of multiple Spriests in a raid/bg/arena.
Imp. Vampiric Embrace - New spell name/icon. This is a 10sec raidwide buff that will heal up to 3/5/7 people based on dmg done by the Shadowpriest. So basically a nice utility spell on a 3-5min cd. (Vampiric Blood/Call)
Mindflay -
1) Mindflay stacking bonus each tick produces a raidwide effect. My thoughts on this was from the old Surv.hunter expose weakness or a rogues hemorrage.
2) Mindflay glyph for PvE, increases Mindflay's channeled time by 33% and adds a 4th tick. (would this theoretically scale with haste?) The goal here would be to allow a second haste cap and 2 Mindflays in between each Mindblast but they would tick 8times instead of 6 total.
3) Improved Mindflay talent that would allow each tick of mindflay to trigger an effect. (Mindblast cd refresh, instant MBs, refreshed DP/VT etc...)
Shadow Wordeath- new talent Master of Death talent 2/2 Reduces the backlash and cooldown by 100% on targets with less than 15-30% health. (combined with the glyph this could become a decent execute spell)
"The Idea here is to make this a raiding option for Spriests that allow them to go into a "nuke" rotation when bosses are at lower health levels. This can offset our poor burst in the early stages of a fight while potentially allow us to make up for it in the end. It also doesnt effect Holy/Disc priest's use of the spell since its deep shadow talented. SO keep on dodging those polys"
Mindlink - "You and your mindlinked target gain 10% mana/rage/rune/energy and a chance upon casting any spell to gain +5% haste for 10sec. If its too OP for the melee classes then just limit it to mana pools only. (thx to ande9249 of wowhead for his ideas on this)
Spiritform -
1) Your shadow dmg has a 10/20% chance to enhance your shadowform. During this time(10sec) you can cast holy spells. This takes the approach of "should I heal my raid/party or add in some nice burst dps?" You can also add a bonus% chance of proccing this based on spirit.
2) Spiritform on a 5min cd allows the priest to seperate body from spirit. While activated your body will cast the same spells as your spirit casts for 15/25secs. However during this time you take 25/50% more dmg.
Imp. Shadowfiend -
-Talented at 35 pts in the shadow tree
-Requires Spirit tap/Imp Spirit tap and Shadowform
-Becomes a permanent Pet with a cast bar
Shadowfiend Cast bar
-Blackout (20yd stun with 25sec cd and 3sec duration)
-Mindleech/Mindflay (a lesser version of mindflay, 1.5sec cast time, no cd, no channel effect or movement reduction. Can stack shadowweaving and refresh SWP)
-Fade(similar to your imp. fade but it also heals the Fiend slightly and reduces chance to take AoE dmg by 80% for the duration)
-Dark Energy ( a caster/pet friendly buff that last 10-20secs, has a 10min cd, and increases your spell haste/dmg as long as your pet is alive)
You can still have the pet's mana replenishing attack on the cast bar, but it will need a cd relative to its current form.
Summon Shadows - Pretty much a 5min cd summon spell. Shadows will be lesser versions of the shadowfiend but will add a nice burst dps option. It can be a talent that activates with the original Shadowfiend.
Thank you for reading. Id appreciate any suggestions on how to balance these ideas if some of them are good enough in Blizz's eyes to make it in-game.