An idea for a questline

IDK if others have suggested this(I'm sure others did) but a questline I want to see in 5.0 is a "fake WoL"
You encounter throughout the storyline a Hyur in a very poorly made armor who claims he's the real Warrior of Light. The fake keeps bragging about how he accomplished what you did,mispronouncing the names of locations; Primals; characters and dungeons/raids, while saying you're the phony.
After you finish the storyline the WoL encounters the fake near crystals and he explodes saying how he is the real deal and accidentaly becoming a primal.
The trial is very mechanics heavy with the party having to deal with 10 phases, each being an ARR job with their own set of mechanics.
After you defeat him, the fake finally admits his errors and goes to try and become an honest adventurer.
What do you think of this idea? And sorry for errors, I just woke up and English isn't my first language.

I don't think quest ideas are usually that well received on the forums but what you describe here sounds like one of those time-travel/mixed-up-timeline who-is-real kind of "cracks in reality" quests. It's not an uncommon genre and I'm sure there's something like that in the game. If you want a main plot point or a large quest chain revolving around that though then I'm not sure.

We already have this in the game: the first set of Hildibrand quests in ARR dealt with a 'fake' WoL called Humphrey who was going around spouting doggerel about how he fought at Carteneau and stuff.. only to be confronted with the truth and ends up admitting he's a fraud who was not a WoL after all (and to add salt in his wounds, Gilgamesh beat him up and stole his sword). :p

Other npcs at times also commented that anyone who claimed to be an original 'Warrior of Light' envitably turned out to be a fraud or charalatan and thus were not to be trusted, showing that there were those willing to exploit the smallfolk who believed the story and resulting in a lot of mistrust for the idea. It's only the player's character's actions that change that perception (as far as the game goes, the 'original' WoLs never return, unless you are a 1.0 player who got the Legacy story, in which case, you are one of them.).

And as a final coda, the patch storyline for Heavensward involved the Warriors of Darkness (who are basically evil paralell-world versions of 'Derplander' and his friends, the nameless party who are used as stand-ins for the player in trailers and propoganda artwork for the game), and as their name suggests, they are literally dark counterparts to the WoL, although who and what they are exactly is a big spoiler so I won't say anything more.

So this concept is already in the game.

No I think he meant one of those genuinely mysterious, unsettling, "cracks in reality" kind of quests that make you wonder, even after it's concluded, whether something was seriously wrong. Just wrong.

You mean a 'Tomato Surprise' type scenario? Again we already kind of have that with the DRK Job quest line (big spoiler)

the Job quest trainer 'Fray', is eventually revealed, by the end of the level 50 quest, to actually be you, or rather, your character's dark side, given form by the DRK Job crystal and your character's suppresed frustration and resentment towards how they were treated at the end of ARR's story (thus effectively they're sort of a 'fake WoL'). At first Fray's comments and attitude seemed fine, but as the DRK Job quests progressed, things seemed a little... off. When 'Fray' starts attacking other npcs verbally, when it seemed like Fray was the one talking, the response from the npcs is that you were the one yelling at them, insinuaiting that 'Fray' wasn't even there. And finally, by the level 50 DRK quest, 'Fray' is revealed to be dead all along and his corpse has been simply reanimated by your dark side, 'Esteem' (which is literally an evil you in every way, right down to having your character's face!), who proceeds to attack you, resulting in you having to put Esteem down and defeat it, and ultimately supress your inner darkness.

So again we kind of already have this scenario in the game. It's possible another such storyline might find it's way into the game, espeically with Shadowbringers lauiding us as becoming 'a Warrior of Darkness', but it remains to be seen how that would happen.