Xe'ra isn't Arcane. Naaru are solely of the Light:
This is from Chronicles is what you can consider to be the bible of the Warcraft Universe, a textual and visual confirmation of the lore that Warcraft is going to stick by and rectifying it. So yeah, she's simply Light and also those that follow the Naaru like Draenei whether Paladin, Priest and such that are directly in the Naaru's shine.. They don't have Arcane in their arsenal. That's why Balance Druids follow Elune have both Solar (Light) and Lunar (Arcane)... Meanwhile those that follow Naaru only use Light.
As for the Prime Naaru, she is the first Naaru made when the ordering of the Cosmos came. I don't think any other Naaru will really take the mantle of the first because there can only be one 'first'. As for the terms of power, it's not really conjunctive to say because she is a Prime Naaru that she is automatically the strongest - after all, we only had her Heart within the Class Halls. A few have died too, those that died could have been stronger but if they were made by Elune, much like the Dragonflight or Titanic Watchers - they only have a proportion of power given to them.
So yeah a greater one may exist but they aren't necessarily mean cosmic bodies, Titans in Chronicles were confirmed to be Celestial beings, planet sized too. Azeroth is also a Titan, one strong enough to defeat Sargeras but is nascent, with the planet or cosmic body being somewhat of an egg.
From that, the theory that the Sun is a Naaru isn't going to happen. Firstly, you hopped onto the boat given to you that Naaru may be Light that Elune made, and because the Sun appears in the gameplay style therefore is connected. Well, Druids don't follow Naaru, not even Taurens, Taurens as you rightly said follow An'she and Mu'sha from the Earth Mother... and they follow Elune. Secondly, from a visual prospect, straight from the Chronicles bible, the Naaru can't really be a symbolic Sun feature. On top of that they are displayed distinctively. Naaru look like a bunch.... of wind chimes and have done several times over, over several expansions and even timelines... You also have to bare in mind if that's the case then thing's like Mu'ru appearing and being made darker is incompatible, also Sargeras killing them too from the many planets/solar systems he's already destroyed. Fact is, there isn't that many nor has that kind of thing come to pass...
As for Illidan, I didn't really need the whole Light and Dark epitome to conclude he is pivot to the end of Burning Legion. I knew something was up as soon as 2011 in Blizzcon when Chris Metzen himself said his body wasn't in Black Temple. It was a bonus sure he was a person of Light but it seemed to me, Illidan from the very beginning including in Outlands, was simply "fighting fire with fire" and amassing an army to parallel the onslaught capabilities the Burning Legion had, I did summarise that Bolvar would come into play both being agents of Burning Legion too but Bolvar has an endless army supply as you could get but they've evidently saved that for another adventure. Anyway, We just were led and assumed it was to fight against Azeroth though numerous of times he banged on about Burning Legion, fought it many times and so on. Ironically, Burning Legion being with the realms of Fire magic and Illidan forsaking, Nature, Arcane, Frost and Fire magic as a Druid and Mage to become a Demon Hunter in a gameplay sense.
Also... Druidism was later exposed to other races like Taurens, Night Elves and Elune pre-date them by thousands of years as much as Trolls did before Night Elves came from Elune's transmogrification over the Trolls. Trolls also worship the Loa or Wild Gods better known to the Night Elves, the same beings that pop up throughout the Night Elves duration including Malorne who mated with Elune to make Cenarius the patron of Druidism. And to avoid confusion... Malorne and Cenarius pre-date the Trolls.
The reason why it appears odd is that for years it was rumoured Night Elves came from Trolls but wasn't confirmed until now, but with the addition to the game it would have made that suspicion very much true. Back then, they didn't want to fan the flames till they were ready I suppose. It's why Taurens appear from the game to come before Trolls but they don't. Trolls were on Kalimdor too which means Moonkin was near them, and therefore taught the Taurens later who became allies with them... Just as much as Night Elves taught the Worgen near them when allied.
When it comes to it you can say Taurens have a religion and not strictly going to be true to the actual ordering and purpose of things... For example An'she being the Sun, there isn't any physical actuation on it like Elune did since before you had the reality of Cenarius walking around and Malorne appearing more than once like in Cataclysm. Also the right eye is called Mu'sha not necessarily Elune herself, it's just referenced a lot for people as it seems the same... Like Odin, Zeus, God, and more being older, white bearded men in charge...
That also said the Earth Mother in that religion is pretty much likened to Elune herself just in a mythological fashion, as the creator of land... well Elune created a new race, new beings and so on...
I find it perplexing you state things are outdated yet cannot see how relevant it still is, how it ties together and at the same time believe that Legion was the pivot to everything when it hasn't been.
You're simply staging your argument as I said on personal cosmetic and aesthetic agenda's with narrow attempts of making it purposeful based on selective reasoning. When you look at the entirety of lore and how pretty much they all support the concept of Moonkins... I feel there is a tremendous lack of awareness considering it is very well supported and even before Legion came. You preach outdated-ness yet Legion as I said even built upon it despite your reasons... On top of that, it has had continuous gameplay which stuck to the very principles of Balance from the get-go and consistent foundations included that have dealt with it for years and not just on one platform or format either.
I have spoken a lot about the actual relevance Moonkin and their cousins have to the ideals of Balance and how now it makes much better sense then it ever did. That was the purpose of Legion, to finally acknowledge EVERYTHING and the liberate a culmination of decades long lore in the game, to confirm so many secrets and suspicions players merely grasped... The down side is barely anyone stops to read and take it in when it's all in your face... I mean for a long time players thought "why the hell is Azeroth so fucking special?!" well... it's not an Old God but it's a TITAN!
You should just love up that Glyph for the time being as Blizzard will most likely add more in the future like they did with the Warlock Pets because I'm not sure but I don't think Owl Bears will be going anytime soon.