TLDR: The more you discuss what could be changed in vanilla, the higher the chances are something will and you end up with LFR, 4k graphics and ElvUI. (listed these things as things which did not exist in vanilla, not that they are bad).
I find it quite interesting there are so many threads on this forum discussing possible vanilla changes. I have been playing since vanilla and still do every expansion. I love retail, I appreciate WoW's past. I have nothing against bring WoW: Classic - the more content there is for people the better.
However, if the goal is to preserve the authentic vanilla experience, outside of bug/crash fixes, nothing else changes. From classes to bosses to drops to systems - everything remains as in vanilla. If you attempt to discuss changes, authentic goes out the door. The only QoL changes must be bug fixes and nothing else.
Just have a look at the list of threads/polls now on this page:
"Poll: Vanilla WoW Yay or Nah to the new graphics?"
22% want new graphics
"Poll: What changes to classic would you be okay with?"
35% want modern addon/lua/macro support
34% want modern character models/animations
34% want modern graphics
32% want modern interface
15% want nerf to known gold farming spots
20% want class balancing
22% want spec viability
38% want auto loot as an option
30% want auto area loot
13% want quest helper
9% want premade dungeon finder
26% want stacking soulshards
22% want debuffs raised/removed
12% want limit stacking of consumables
17% want more challenging bosses
17% want guild bank
37% want no changes
"QoL needed for an amazing classic experience"
"Poll: Should Talents be Class-Specific rather than Spec-specific?"
"A change classic servers absolutely needs"
"Poll: Would you prefer no updates or updates?"
"Why WoW: Classic Needs the Tweaks"
Now, sure, some of these threads are clearly trolls and bait, but you realize Blizzard will poll the COMMUNITY. That means EVERYONE, people who love Classic AND people who HATE classic will vote. People who have never played Classic will vote for features they never experienced first hand.
While the Classic community is quite vocal and has been asking for Classic for a long time, the casual WoW retail players are a lot more. So how would you select whose votes matter?
This question is really important when it comes down to the intricate details of vanilla, such as the detailed poll I posted above in this post illustrates - stacking soulshards? debuff slots? spec viability? You can see a basic poll on mmochampion results in such high %s for things which are unthinkable for vanilla.