Anything but legendary weapons, please. Nothing breaks the old PotCO feel more then those for me.
Sailing content is direly lacking:
Worthwhile and interesting upgrades (easiest one to think of, variants of current R4s but trading the special broadsides for additional stats)
Additional Ships ( An important part of this is that, unlike certain added weapons, not to entirely wipe the purpose and niche of the pre-existing ships by virtue of simply having the higher statchecks)
Easiest to add being the Sotl/carrack due to already having material to use to implement them, as well as both having been in the "old game" and therefore having more of a history and basis to be added. Though the Sotl could probably use something similar to the BP/Ravens Cove story quests to unlock it first, I remember reading something about needing the 3 "Admirals" swords to get one I quite liked.
Theres also the undead war frigates/war sloops, re-texture them and fix the "damage" on their models and you could make variants of the war sloop/frigate, which could be as simple as a different model with the same stats, or as far as making them have different stats and acting as an alternative to the current selections. Ie the Undead war frigate, being much smaller, could be a faster but less tanky war frigate but maintaining a similar profile and firepower.
All these have pre-existing resources in-game, that'd make re-purposing them for use easier then designing a new ship from scratch and wouldn't break the "feel" of the old game, as we are used to seeing these on the open seas before.
Sail Upgrades could do with additions, immediately 2 come to mind of percentage increases to speed/turn, I'm sure theres others that could be done such as acceleration.
I can't even call the current NPCs as having an "AI", being such that the deck guns of enemy ships may as well not even fire. The ships themselves merely straightline until X range and turn to shoot untargeted broadsides over the top of your ship, so much so that I have successfully dismasted my own war frigate by positioning next to battle-royale to take explosives onto the sails. Enemies that actually fight back would be a very big change for the current sailing situation.
Personally I'd like to see the Infamy mechanic significantly reduced, and in the case of ships that only target undead vessels non-existent, doing a treasure fleet with Infamy sucks, you can hardly kill half the ships before warships run you down, hardly the intended experience when the treasure fleets were originally introduced. Along with the above suggestion, this would balance out it taking longer to reach the "Challenging" stage by making the current enemies more then a turkey shoot.
While I don't really think that sailing is in need of this sort of content, I can certainly see adding more "sailing exclusive" loot to the game, it'd be one way to bring back the old "Norringtons Spyglass" item, or maybe add new sailing items that take advantage of sailing customisation, offering a mix of cannon/sailing skills but with less significance then a dedicated item of its respective class.
Clothing is another option, but a poor choice imho due to the subjectivity of appearance reducing the potential effect it may have.