Just a few suggestions from a mmorpg veteran.
Keep in mind the psychology of the gamer. What makes us come back?
1. Customization. What causes a gamer to become attached to the game? Normally one becomes attached when there is something tangible to become attached to. This could be numerous of things but player possessions (collections/treasure/pets), housing, claimed land, production, a customisable avatar come to mind here. Many recent games experimented with this and it works! Of course game-play comes first; without awesome combat, exploring etc, you can forget about the rest but stuff that makes you stand out in the crowd (without getting unrealistic) does come second.
2. Good and balanced mix of solo and team-play. So many people, so many different tastes. One only needs to look to classic mmo games who are still going strong today. It's only because they offer it all. I got in to Archeage and liked it a lot but because it's really only fun when playing with an active guild, I left. That.. and I forgot to pay the rent.
3. Economy. Just look at EVE online and copy / paste. There's no shame in doing so. Same goes for the different secs (1.0 pve / <0.5 pvp). Furthermore I'd focus on free enterprise as it would be very 17th century to do so.
4. Role playing. Naval Action begs for good RP elements true to the era of naval warfare. Please don't put pink bunny hats in-game! Instead I think pp would really appreciate 17th century military costumes, ranks, titles, you name it. I'd most certainly enjoy sitting my avatar down at the port's café to show off my self inflicted tattoos and brag about the battles I never fought. And how about putting in some historical characters along with each nation. Maybe combined with some sort of (mission) story line.
5. Emergence. One thing I hate in a lot of new games is that the game mostly plays itself. Back in the day, you had to work really hard to get something valuable. Now it's just: "kill four of those unimportant swampcritters and get this epic great-sword of the heavens". I do hope Naval Action will reward players who put extra effort in it. Emergence for me means rewards don't come easy, just like in real life.
6. First person action? I don't know where the devs ultimately want to bring this game but so far I haven't seen a game which combined vehicle action with first person action very well (Star Citizen is on his way though). But take a look at a game like 'War if the Roses'. Is a combination unthinkable? That would be a game I wouldn't stop playing. A real pirates' life does come with a sword and a pistol.
I realize I'm just shooting away here because I haven't seen a list of goals set by the devs or something and I'm only 2 days in but there ya go.
Good luck for the devs and may we sea-dogs all gracefully die in battle !!!