Now i know this will be a disliked post by all the people who haven't become pirate legends themselves, but remember you will get here too one day!
So as most pirate legends know there isnt much content endgame other than collecting the same stuff you have done for the past 150 levels of rep. So my thoughts is that having a developer team focus purely on making pirate legend content will flesh out the endgame and keep veteran players playing, and give new players more and more to look forward too.
One such thing could be a page out of the anniversary updates book, being a island or sector of the map being locked specifically too pirate legends, here the PVE section will be tougher, with much more mini bosses (Captains/skeleton lord), and end of voyages having a final boss earning large paydays (Ship battles, Sea monster encounters).
Another thing that i would love to see is daily challenge and weekly challenges, now for new players they would earn gold and rep for certain factions of their choosing, but exclusively for pirate legends they would earn pirate tokens. Now these tokens once saved up enough (Say 10-20. 1 gained per challenge) can go towards the pirate emporium (Which i take will be the real life money in game store) Now i know this may seem a bit off, taking money out of Rares hands, but A: It would make veteran players keep playing which they probably want, and B: Looking at other games like League of Legends, they have characters that are both Paid with real money or earn after playing for a long amount of time.
Another thing that i would like to see is a Vault. Now this is a island in the centre of the map, and this could or couldn't be exclusive for pirate legends, but here you can physically see your gold piling up in the middle (Huge pile), and surrounding this pile of gold is 12 barrels, each barrel represents a real month, and after a month the pile of gold goes into one of said barrels and depending on how much you earn't the more full it becomes. After the 12 months your hard earned gold earns you a exclusive trophy for your vault and some other neat things (Exclusive weapons, cosmetics, pets etc). This could also be used as a leaderboard of some kind showing how much players have earn't by them entering your vault.
And my final idea is weekly bosses. Every week a boss (Could be a variation of kraken, megladon or skeleton lord) will spawn on a island on every server, and if you manage to defeat it you gain a trinket (Meg, giant shark tooth. Kraken, Stuffed tentacle, Skeleton lord, sword/bone/skull). These trinkets can be used on your boat (Like sails, anchors, cannons etc) and can be swapped out. Being able to visibly see your accomplishments and showing others your adventures.
Tell me what you think below! And please keep it friendly!