(Most of this a bit off topic, but i got carried away while writing it)
It's super cool idea. Those of you who played AdventureQuest Worlds probably remember how the devs in game characters were key people from the lore, which i thought was very cool. The sad thing is that(As far is a understand) there's different seas(servers) to limit the amount of ships on one sea. This would mean that you would have to be pretty lucky to land on the same server and actually encounter these "Legendary dev characters". Not that it's necessarily a bad thing, i'm just afraid i would never encounter them :)
Another thing along the same lines tho. In the progression video it's mentioned that the most infamous pirates "will play in important role in the golden pirate age to come"(Something like that). This could mean that Rare plans on letting some of the most infamous pirates(Maybe someone with huge amounts of faction proggression?) get a special status in the Sea of Thieves. The quote from the video allows for a lot of speculation though. It might not even be suggesting any in-game event. Though i do think that there's some potential when it comes to the "Legendary pirates" concept.
I think it would be amazing if we were to see some of the most hardcore SoT players act as bosses for certain events. Imagine for example an event where the gold hoarders would have to ship some of the massive amounts of treasure the pirates of SoT have gathered for them. This event could play out in the form of some super strong galleon(Sort of like a raid boss) filled with sparkling treasure, and manned by those players most trusted by the gold hoarders(Players who have done A LOT of gold hoarder voyages). And then other players trying to team up to sink this ship and take the treasure.
Similar things could be done with the rest of the factions. The merchants guild doing a huge transaction with some other merchants. And pirates trusted by the merchants guild would be bodyguards(With weapons stronger than the regular. Again, as sort of raid boss characters) to ensure the transaction goes smoothly, and doesn't get interrupted by greedy pirates trying to take the gold and trade goods for themselves.
And of course for the Order of Souls there could be some of their most trusted accomplices(Players, in case you didn't where i was going) doing a spooky ritual in a cave filled with a skeletons. Other players would then have to team up to plow their way through skeletons with enough bananas and ammo left to overcome the vicious pirate crew inside.
These are just few examples. Even the whole concept is just one of example of how pirates could feel "Legendary". I hope that something similar will be added to the game though, maybe in the far away future. The reason for this is that i imagine that even though the voyages and the game overall seems like a lot of fun. But i can't imagine that you can keep playing indefinitely without the voyages getting tedious at some point. If there's recurring events where players take the place of what could've been NPC's that gives people something to do at the VERY end game. To be honest i think most people even after clearing ever corner of SoT would come back a few times a month/week if it was because they got to be a raid boss :)
Let me know what you think about these ideas. And please share any ideas you have for a way to feel that you're getting that "Legendary pirate" status. I'm very interesting in discussing this :)
Lots of love, Flade