That’s a lot of guessing for something the game doesn’t make any effort to portray, which is again part of the problem. At least to me.
Maybe they’ll rectify that in the WSS (still trying it out). We’ll see.
Because right now, based purely on the artwork and the trailer, it looks like its mostly going to be a Anduin/Thrall/Alleria vs Xala’tah story, with our characters just being there for the ride.
With lightsaber battles and using the force powers of Azeroth!
Didn’t read the long post by the OP in its entirety.
But if the main point was reflective of the title…
It’s a common troupe in fiction story telling especially fantasy and sci fi genres.
The main character or characters are the champions the heroes…the SAVIOURS OF THE UNIVERSE!
Sorry got carried away.
It’s just in a video game we/us …the proverbial “you” plays that character or characters.
It’s nothing new and blizzard / wow is hardly the originator of the concept.
Not even close in fact.
Read through any vast numbers of games descriptions or better yet play them your character is the hero or champion.
I don’t get why people get so wrapped up in “I don’t want to be a hero” …it’s a game just have fun and stop overthinking.
I mean yes our characters are kind of a mary sue/gary stu based on “everyone loving us” however we don’t meet the biggest qualification of being good at everything with no effort. I mean that’s what the entire leveling journey is and especially if talking early WoW where you start as a weakling nobody and rise to the top.
Now if your character buys a level boost then they 100% fit the mold as they now are insanely skilled and powerful with no real reason and everyone loves this person that should essentially be a nobody as they haven’t made a name for themselves.
And also raid ready! 
Lotta good points in here, but i think ultimately it comes down to the squeeking wheels. The squeeky wheel gets the oil, or for comparison, the whiney player gets the attention and fixes. The writers should quit paying attention to the whiners, and worry about maintaining the hero/champion narrative. But that would require the majority of players that feel being called champion is better to actually speak up and cry louder than the players that cry “I’m not being treated special enough! I’m not over powered enough! My healer can’t do as much damage as the tank!(granted shadow priest is long over due for a competative rework more than any class in my opinion), My character can’t just walk through a dungeon designed for 5 players!” On and on and on. But like what has happened countless times since the release of vanilla, the crying players push the developers to the point of the game is a walk and way too easy. So it starts working it’s way to being a challenge again. All you can do is when they send out the surveys on what you think of the game, make the time to fill it out and let the developers know that at this precise time the game is just perfect. For every cry baby that puts out a power gamer whine, all of us chill players need to step forward and remind them that the screaming baby in the corner doesn’t represent all of us.
Sorry that got a little long winded but there you go.
Hey blizzard, quit treating us like a spoiled child that needs a cookie for everything, and quit listening to the players that scream the loudest.
Not quite…
To be fair, the ‘Chronicles’ books were FANTASTIC for addressing that to some degree by making ‘The Heroes’ – as a collective, canonised in the story for defeating famed mobs throughout dungeons, raids and events of adventurers aiding important feats & discoveries. 
What is a Mary Sue?
I almost feel this is a slight to us.
Congratulations on saving everyone. Could you serve tables?
Dadghar is sad can you get him a drink dear?
Can you stab the sprites pretending to be decorations? We really don’t want to give them a break.
The story assumes your character has been around since day 1 of Vanilla.
The idea that’d we still just be random adventurers is more ridiculous than any other trope.
I would say I feel more like I am watching the avengers, and not really participating in anything.
Ahh yes a being called a champion through out the expansion pack but at the end you get one mighty final quest and that’s being a waiter at the big party.
Something like…
Serve
0/1 grab one apron
0/23 Thirsty patrons served
0/15 Hungry patrons served
0/1 Make sure Slorn Fetersun gets his favorite Old Number 32 liquor
0/6 Malfunctioning Insta-Drink Mixer 6000 machines disabled
The Kingdom of Heaven type ending. You were a “Champion” but then you go back to being a mere blacksmith,engineer,bartender,waiter,janitor,town begger,garment district salesmen etc. until needed again of course.
Blizzard’s trying to do SPRPG in a MMORPG. – Literally every Paladin had Ashbringer in Legion. 
Not to mention they’re writing team is staffed with nothing but Fanfic writers.
This is why ‘your’ story is so convoluted.
Sums this post up precisely.
I like this idea purely for the comedy it brings haha. Imagine the quests " Hey God Emperor insert player name “I need you to go pick nuts from 10 dragonkin poops for my research.”. Typical work for a God of azeroth.
Yeah the way FFXIV handles it is literally “go gather your most trusted companions, we know you have a whole team for this kind of thing,” for some of the raids, and others that are thematically “individual” fights they’ll either give you a 1v1 encounter, or if it is party content it’s disguised as “calling out to all of the splinters of your soul.” (Which is a story thing established elsewhere, not solely for the purpose of these fights). You’re referred to as “Champion of Eorzea,” but your group exploits are never your own. Always referred to as “you and your fellow adventurers.”
Im of the reverse opinion…
I killed the Lich King and Death Wing, i went into the afterlife, walked the Maw and returned…
Now you want me to get your cat out of a tree and corral your lost sheep for a few gold?
Less splinters of your soul but more “pulling likeminded allies from wherever to aid you in your time of need”.
A bit less cool than implying each player is part of your soul, but it works lol.
I think it’s kind of crazy how everyone loves the player character, and how we will always win every fight we get into.
Mary Sue? Yeah, probably.
Lazy writing? Yeah, probably.