In this post, I completely disregard PvP issues, and talking from a pure PvE perspective. Feel free to imagine any PvP talent that would make up for these changes. I played Discipline in all of MoP & WoD, doing mythic raiding. In my opinion, all iterations of Discipline had some good parts, with most of them found in the Legion one. I don't generally raid in Legion but only casually play instead. Here are my thoughts and suggestions:
The problems of Discipline:
(1) Shield Spamming as your best (and really the only) way of healing must had never happened, and should never come back. Ever. For me, this includes the removal of PW:S' cooldown. The solution to this is to always make sure that PW:S has a cooldown.
(1a) PW:S should always feel meaningful, and being able to spam it constantly took away from that. The Legion version of mostly using it on your tank every time it was available was not of my liking. It felt empty and ultimately a wannabe mana regen tool if you had taken Shield Discipline: without choice, except for those times when you would shield someone soaking.
(2) Discipline is too mobile with instant PW:S and moving while casting Penance. You are not going to cast Shadowmend in raids and the only thing that requires the Priest to stay and channel would be PW:R. Feathers are plentiful, but by no means think that I am suggesting that Priests are anywhere close being top dog in mobility. VERY far from it, especially after the movement speed bonus reduction on the Feather.
(2a) With spammable shield, and outside of Smiting, I don't see why the priest would stand to cast PW:R, unless it's an emergency or they were preparing for Light's Wrath, but that is not present in BfA.
The goods of Discipline:
(1) Most positives come from Legion: Light's Wrath was a fantastic ability. It hit hard, it healed hard, it had an awesome animation, and the ridiculous numbers it could provide when heated up (not present in raids) was phenomenal and immensely satisfying. Add to that the synergy satisfaction you got from applying Atonement as much as possible to shoot a huge Wrath, and you have a wonderful ability.
(1a) Light's Wrath MUST be a cooldown available to Discipline in BfA. It added complexity, satisfaction and execution requirements to the spec.
My suggestions transitioning from Legion to BfA:
Things that should no longer be in our spellbook:
Power Word: Radiance is removed.
Shadowmend is removed.
Plea is removed.
Power Word: Barrier is removed.
The following abilities are changed, replaced and reworked like so:
Clarity of Will becomes baseline, replaces Shadowmend and is reworked into:
Clarity of Will
4% base mana
2 sec cast
Shields the target with a protective ward absorbing 500% spellpower damage and healing them for the same amount. The ally will take [(500% of Spell power) / 20] damage every 1 sec, until they have taken 500% of Spell power total damage from all sources, or leave combat. Applies Atonement.
Light's Wrath is retained, but its cooldown is increased to 2 min and its baseline damage is also increased by 25%.
Luminous Barrier (the new talent for Discipline in the BfA alpha) takes Power Word: Barrier's place and becomes baseline. It is reworked in:
Luminous Barrier
3 min cooldown
No cost
Shield all allies within 40 yards of you for half your Power Word: Shield's value for 5 seconds and applies Atonement to them for 5 sec.
Rapture is reworked into:
Rapture
1.5 min cooldown
No Cost
Does not trigger the GCD
For the next 12 seconds, your Clarity of Will costs half as much mana, has its cast time reduced by 50% and becomes castable while moving.
Power Word: Shield is reworked into:
Power Word: Shield
40% of base mana (up from 2.3%)
2 sec cast time (up from Instant)
20 seconds cooldown (up from 0 or 7.5 or whatever)
2 charges
Range: 40 yards
Shield yourself, your target and 2 other allies that are in a 15 yard range of your target for an amount equal to what PW:S shielded for after the Highmaul first nerfs. Shields created by this ability apply Atonement for 60% of its normal duration.
It will favor players without Atonement, then players with Atonement, then non-players as Power Word: Radiance does now.
The following talents are reworked and moved elsewhere in the talent rows:
Shield Discipline:
When 3 or more shields you create are fully absorbed within 2 seconds of each other, you reduce the cooldown of Light's Wrath by 15 seconds.
Shadow Covenant is reworked:
Your Power Word: Shield spell shields 2 additional allies, but if the shield is not fully absorbed, any remaining shielding value is tripled and applied as a damage over time effect on its targets.
Last notes:
What I want from Discipline is for its abilities to feel powerful and meaningful. Move far from spamming shields and allow interesting gameplay. While I am generally not a huge fan of Atonement, I can certainly see the appeal in it and what it can offer and I am all for it. In my opinion, these changes achieve the following:
(1) Merging abilites and relieving clutter: Shadowmendt, Plea, PW:Radiance, etc.
(2) Providing meaningful shields that are not spammable, but are controlled and lead to Atonement healing, without going through 15 seconds of spamming Plea to your raid just to be able to use Light's Wrath.
(3) Emphasize the timing shields need to be applied, by taking away spammable shields (PW:S no cd), and making the new Clarity of Will's nature answer to duality: both healing & shielding. That way you waste healing by mindlessly spamming as in WoD AND lose on the mana to healing conversion when using it on a full health target.
(4) Rapture allows for much needed mobility in M+ and a good way to play the Plea game for those that liked it. I personally dislike instants in healing.
(5) Power Word: Shield turns into a real Power Word, allowing for key moments of shield placing and fully moving it away from blanketing the whole raid (always talking from a Mythic raid perspective). Account that the Priest only chooses one target to shield plus themselves, and the others are chosen by standard smart healing rules.