Assassin's Creed Black Flag was a masterpiece in many ways, primarily on how it claimed the title for most in-depth and complete sailing game at the time. Years later some of the same developers have announced Skull & Bones, the sailing spiritual sequel without it's Assassin counterpart.
But Black Flag was a lot more intricate than just a sailing game, and the Assassin portion added a huge design element to all of its components. Let's take a look at Everything Right with AC: Black Flag and use it as a guideline and lens which to look at Skull & Bones through.

• The Spyglass

A simple tool yet a core mechanic of AC4, the Spyglass allowed players to scan the horizon manually for a number of reasons.
It could list the cargo type and quantity within ships.
It could zoom quite a bit.
It could highlight and mark something on the map, giving it a waypoint, such as ships or locations of interest.
It could identify the nation which a ship's flag was flying under.
Many of these features were very important, knowing which cargo a ship held allowed players to hunt for specific resources needed  to upgrade specific parts of the ship as desired, highlighting things on the HUD was a quality of life mechanic that made traveling and self-designed progression easier, as was zooming, and noticing which flag the ships held allowed players to maneuver between battles and prey on them by using conflict between nations as a means to an end, like a true pirate.
All of these mechanics being so useful allowed the player to constantly have reasons to use the spyglass, which added to the captivating experience that transported players into the game, giving it a "real sailing/pirate-like" experience.

• Seamless transitions into Ship Boarding Battles and Exploration

Being able to quickly lock the player in a Boarding Battle without loading screens or transitions allowed for ship hunting to be quick and effective and less repetitive, also allowed the player to engage in multiple battles at once giving the protagonist a real sense of power and presence not just through the story or dialogue but also in the gameplay itself, since the player was prevented from being damaged by other ships while boarding, which also added to the cinematic experience that AC4 was. As well as allowing the player to dock pretty much anywhere in order to go on shore or explore a small town and check islands and hidden caves and anchor down for diving, all without a loading screen apart from main (and much bigger) cities, offering a really immersive world and proves how a good sailing experience needs more than just good sailing mechanics.

• Repairs and rewards from Ship Battles

After each boarded ship the game would ask the player what to do with the prize, offering to use it to repair the ship from damage done in the previous battle while also rewarding the player with some ammo and loot allowing the player to continuously hunt as desired only being required to stop every now and then in order to fully resupply the ammunition cache and perform upgrades. That was important to ensure a minimal amount of hassle during such a core element of the game, which also avoided causing a repetitive and "farmy" experience.

• Travel Speed

The ship had multiple levels of speed and handling, but one special addition was the Travel Speed. It was only available outside of Storms or Battles and allowed the player to move just fast enough in order to cover larger distances while still encouraging exploration as opposed to Fast Traveling, and the camera was placed in a very cinematic angle as well which provided an amazing sailing experience.

• Shanties

Sea Shanties were one of the most memorable elements in the game, the development team truly outdone themselves when it comes to their production and composition. The combined chorus of voices provided a very intentionally detuned and sometimes out of sync atmosphere very much in character with what a Pirate ship would sound like, and the shanties themselves were an incredibly engaging soundtrack to sea travel that was a key component on its immersive aspect. In addition, players were able to skip songs and mute them as desired.

• Ship fighting mechanics

The game had plenty of ways to engage ship combat which diversified the gameplay and added optional strategies, from using the long range and high damage but slow firing Mortars to using the Chaser Guns that only fired Chain Shots from the front of the Ship in order to damage sails and reduce an opponent's speed, from the Fire Barrels deployed at the back of the ship as a form of landmines to the old faithful and expected Broadside which used 2 different ammo types for long range and close range complements. The game offered several ways to approach combat and carry progression through upgrades.

• Multiple additional gameplay elements

AC4 managed to remain interesting for dozens and dozens of hours by also offering different activities for the player. Such as Harpoon Hunting locations with fully unique mechanics for hunting whales and sharks and other game, a Diving Bell which allowed the exploration of deep water wrecks for treasure and collectibles and resources while avoiding the dangers of the ocean and even utilizing stealth mechanics to avoid sharks, and different occurrences in the overworld such as Forts to take down and conquer, Legendary Ships, to battle Hidden Caverns, Treasure Hunting, Assassination Missions, Templar Hunting missions and a few others still.

• The Assassin Gameplay

Black Flag used the core Assassin's Creed gameplay to its advantage, being able to climb the many ropes and rungs of the ship during Boarding Combat, utilizing patches of seaweed to stay hidden and stealth away from deep water predators, infiltrating plantations and werehouses to dispatch slave owners and collect bounties among other uses allowed for the overall sailing and game experience to feel deeply connected, engaging and full of life.

• Ocean Simulation and Stunning Visuals

The ocean is impressively well done in AC4, the waves feel heavy and can drastically affect the gameplay by creating cover during battles and altering maneuverability options, simulating storms and variations in weather and tide which the game uses as scripted sequences during certain mission but also completely random during encounters and ship hunting. The visuals themselves are spectacular, specially for 2013, with foliage effects that sway in the wind and create realistically large patches of grass for stealth and terrain coloring as well as many unique buildings and structures used for the parkour gameplay such as each ship containing several layers and levels of ropes and beams to cross and explore.

• Ship and Player progression

While the player's effectiveness in combat stays generally the same stats wise, the game offered meaningful progression in the way of upgrading Cannons and Armor and things like Storage which changed the way players would approach combat slightly as well as increasing the ferocity of the sound produced by the Cannons with each upgrades and teaching the player different ways to utilize the mechanics in combat to approach different kinds of challenges such as multiple ships, Battering Ram ships, long range mortar fire. The game offered a decent amount of customization in terms of Sail types and colors and Clothing Outfits but this is an area that Skull & bones can definitely improve upon if it would offer a greater variety of playstyles or visual changes to ships depending on which upgrades the player would specialize in and things of the nature.

• The Story

Last but not least, utilizing the classic Assassin's Creed theme, the game offered many locations and characters of interest from history shaping and recreating them in original concepts that offered an incredible life to the world of the game, motivating the protagonist to evolve in order to reflect their surroundings as the player's actions and time spent in that world start to broaden the scope of it and create a truly epic adventure across multiple timelines with some of the most memorable and likeable characters in the Assassin's Creed franchise.
A true love letter from the developers towards attention to detail, meaningful dialogue, emotional experiences and everlasting memories.