8 ppl liked the other post I made about FIre/Frost Mage tuning, so Imma do the same for Priest!
I actually play all 3 specs so I can express my opinion on all 3 of them:
I only had some really minor ideas for Shadow and only for its 2nd row of talents so I decided to omit Shadow entirely.
For Discipline and Holy Priests:
1st row:
[Discipline]
Castigation: Also make it reduce Penance’s CD by 25%.
[Holy]
Enlightenment: Increase mana regen by 20%, instead of 10%.
Trail of Light: 45% of the healing, instead of 35%.
Renewed Faith: Healing is increased by 15% on targets with Renew. Direct heals increase the duration of renew in the target by 1s.
2nd row:
Angelic Feather: Make it increase movement speed by 50, from 40%, and make it dispel (magical) movement impairing effects. Body and Soul simply outclasses it because you can reliably use it anytime you need a speed boost, no CD, more overall speed, easier to buff allies and they don’t need to walk through to get the speed boost.
[Discipline]
Masochism: Additional effect: Allies affected by atonement reflect 10% of all damage they take to you. (Replacing “and reduces all damage you take by 10%”)
[Holy]
Angel’s Mercy: Also allow it to be used on allies.
3rd row:
[Discipline]
Shield Discipline: Increase Power Word: Shields’ absorption by 25% and reduces its mana cost by 35%.
Mind Bender: Allow it to store up to 3 charges. Yes, you could have 3 Mindbenders at once if you used them in a row. and make it regen 0.8% mana on attacks, still less the normal Shadowfiend’s 1%.
Power Word: Solace Double its damage so it feels good to use, and make it generate 4% Mana (2,000 out of 50,000 mana), from 1% (500 out of 50,000 mana).
If you think this row would increase mana regen by too much, then the mana costs of all spells could be increased slightly to compensate.
[Holy]
Cosmic Ripple: Double its healing.
4th row:
Shining Force: Reduce its cooldown to 30s, from 45s.
5th row:
[Discipline]
Sins of the Many: This increases your healing for when you need it the least, and everyone knows it is not the best talent for healing, nor it is trying to be. The 12% damage boost is just not enough, that is why I think it should be increased by 24% reduced by 4% per ally with atonement, including yourself. Down to 4%. I believe this would make it worth picking for when your healing is fine and you just want some extra damage; or solo content.
Contrition: Increase its healing by 50%.
Shadow Covenant Make it uncapped, allow it to heal all allies in range and not only 5. This would only make a difference in Raids.
[Holy]
Binding Heal: Increase its healing by 50%. I know a lot of stuff on this post is around a 50% increase, but these talents are honestly tremendously lacking and need a HARD boost.
7th row:
[Discipline]
Lenience: 5%. Just make it 5%.
[Holy]
Light of the Naaru: Instead of its current effect, make this talent passively reduce the Cooldowns of your Holy Words by 40%. Apotheosis is currently just better for mythic+ for the on-demand functionality, and Holy Word: Salvation is our Raid cooldown… SO Light of the Naaru needs some love.
And that’s it! 
Might do DH later…