Hi!
Rundin here, priest more or less full time for 20 years. Mainly Holy but “there can be no light without shadow” 
I hope Blizzard reads these or that someone points me a direction to send this further.
TLDR 1 (RP/Lore): You are in danger of loosing the class feel in favour of a spec feel once again, please dont repeat your old mistakes that you actually fixed in shadowlands after releasing classic by re adding our core cross spec class spells!
TLDR 2 (Mechanicly): Holy needs a full range instant filler heal while moving and an open world world damaging “pull spell”. Shadow needs an instant filler when moving and having no available procs.
(PW: Shield, manual renew and shadow word pain need to stay cross spec!)
Firstly, this is not a complete “dont take my favourite spells” post. I actually really look forward to the less buttony more focused classes and the “simplification” has my full support. Most of your planned removals are sound choices.
However… you are opening an old door to past mistakes. Starting in pandaria going on through legion and bfa to be fixed in shadowlands you admitted to “focusing too much on the spec and loosing the feel of the CLASS”. And you adressed this by re adding some cross spec core class spells for everybody. For example shadow word pain. Hunters mark. Corruption. Rend etc and many more.
The spells we start with or get before level 10 that define the feel of our class we need forever! I remind you of these thoughts: https://worldofwarcraft.blizzard.com/en-us/news/23310980/shadowlands-class-updates
In your own words:
“even though a Holy Priest excels in their connection with the Light, they are still able to harness some Shadow magic to deal with foes”
“Likewise, even though Shadow Priests focus on harnessing Void energies, they are still able to draw on the Light to protect themselves.”
Please dial back slightly and keep these typs of spells. Even if you gotta put them in some sort of “Cross spec flavour spells” cathegory to avoid confusing newer players what to put on their bars!
I remember on a 4:3 fat screen back in the days on world of warcraft web page before even the alpha was out. I was looking through the classes and most had some old warcraft 3 spell defining them that i recognised. Each class had 3 example spells. Warriors had whirlwind! Mages had polymorph!
Then i came across Priest! Shadow word Pain was right there!! Next to heal and dispel magic i knew from warcraft 3. It immediatly sparked my interest and having been a bit sceptical about not being able to kill enemies while i wanted to heal this settled it for me. “oh i can harm with shadow while saving with light, this is awesome!”
And now im once again about to loose that core defining spell? Please leave it!
Just as holy leans to shadow, shadow leans on holy. both lean on disc which leans right back..
We dont need all of them for example shadow word death in holy but hear me out;
My idea is that shadow/disc get flash heal or renew from holy,
holy/shadow get power word shield from discipline,
and lastly disc/holy get SHADOW WORD PAIN from Shadow!
That being said lorewise, this fixes several issues mechanicly aswell.
-Holy retains a spammable tool to pull mobs while running in open world and old content and so does the other 2 specs. An instant holy fire could solve this but possibly make it op therefor keeping the shadow feel is a better choice
-Holy has a filler to cast while moving when noone is in holy nova range even if the shield is worth only half a flash heal its something! and then it can still proc body and soul more on demand than POM which is always on cooldown
-The above is also true if we get to keep renew instead and that procs body and soul, and honestly a healer without an active cast HOT feels lacking.
-Shadow can still manually cast SW pain in lack of other spells while moving
-Shadow can survive a bit better open world by self shielding
-I dont play a lot of disc but surely a disc without SW:P, PW:S and FH would feel really wierd.
The alternative to pull with offensive dispel magic feels wierd to say the least!
Im looking forward to a less buttony but still very much focused and centered PRIEST! regardless of what it is specialized in!
Thanks for your time! 
/Rundin Graveears
Argent Dawn
20 years faithful player