- 新的海军任务·寻猎：选定的舰队会在指定区域巡逻并攻击敌方舰队。Hunting naval mission: Fleets will hunt enemy fleets in a specific region.
- 新的海军任务·避战：选定的舰队会避开指定区域的敌方舰队。Evasion naval mission: Fleets will evade enemy fleets in a specific region.
- 新的海军任务·封锁：选定的舰队会在指定区域封锁敌方的港口。Blockade naval mission: Fleets will blockade enemy ports in a specific region.
- 新的海军任务·拦截：选定的舰队会在友方海岸线附近攻击正在卸载敌军部队的运输舰队。指定区域封锁敌方的港口。InterceptIntercept naval mission: Fleets will seek out and attack enemy fleets near allied coastlines, focusing on troop transports.
- 新的外交选项·无条件投降：敌人获得一份100战争分的合约，如果他们两个月之内不接受，那么呼吁和平的效果将会变成原来的二倍。The new diplomatic option Unconditional Surrender forces your enemy to send a peace demand for up to 100 WS within two months, or Call to Peace will be gained at twice the normal rate.
- 三个新的间谍行动：诽谤商人团，5年内减少目标国家的贸易力量；破坏征兵，5年内减少目标国家水手和人力的补充速度；腐化官员，增加目标国家的腐败度。Three new espionage actions: Slander Merchants reduces another country's trade power for 5 years, Sabotage Recruitment reduces manpower and sailor recovery speed for 5 years and Corrupt Officials increases corruption.
- 海军任务·猎杀海盗（dlc黄金国也可解锁）The Mare Nostrum DLC also unlocks the Hunt Pirates mission (from the El Dorado DLC).
- 新的Unit View: Added Naval Mission Settings panel that allows you to adjust the behavior of a fleet when on a mission.
- You can now rent out Condottieri to other countries.
- You now press sailors automatically when occupying coastal provinces.
- "Detach Damaged" is changed to "Repair Damaged". Damaged ships will head to port for repairs and then reunite with the fleet they were detached from.
- You can now send a diplomat to conduct counterespionage against a country, increasing the chance of detecting their attempts to build spy networks by 100%.
- Promote Mercantilism: Spend diplomatic power to strengthen your mercantilism.
- Added a Timeline feature that allows players to observe how the world has changed throughout the game.
- A leader of a Trade League, can now create a Trading City from one of its provinces. This new nation will have the Trading City republic government and will automatically join the Trade League. (introduced with patch 1.18)
- Majorly reworked Espionage. Instead of sending diplomats to carry out sustained spy actions, there is now a new 'Create Spy Network' sustained action, where your diplomat covertly builds up a spy network in another country. Spy networks are size 0-100, and espionage actions other than Build Spy Network are now instant actions that cost a certain amount of spy network points when used but are guaranteed to succeed (for example, Sow Discontent applies the Discontent Sown modifier to the target country for 5 years). Spy Offense modifier was changed to Spy Network Construction and determines the speed at which networks are built, while Spy Defense determines the chance of detecting when another country is building a spy network in your country and disrupting their efforts.
- Added a Loot Mapmode.
- Study Technology is no longer an action but a discount to technology cost gained from having a spy network in another country with more advanced technology. Size of spy network and how many techs they are ahead of you determines the size of the discount, with only the largest discount for each category used (so if you get 20% land tech discount from one country and 15% from another, your actual discount is 20%).
- Naval Combat: There is now a chance of capturing ships that would otherwise be sunk, based on Admiral Maneuver skill. (This is in addition to ships with low morale captured after the battle.)
- Naval Combat: The Positioning concept is removed.
- Naval Combat: Introduced "Engagement Width", which is the number of ships that can engage the enemy each turn. This is modified by leader maneuver skill and local/global modifiers.
- New mapmode: States and Territories.
- Added the historical_neutral tag which makes countries less likely to become friends or enemies.
- Admirals now have siege ability which increases blockade efficiency by 10% per point.
- Merchant marine now gives +50% Sailors & Press Gangs now give +20% Sailor Recovery.
- Added mechanics for Sailors, given from development in ports. Sailors are used when building ships and repairing them.
- Added continue button to end game screen to allow you to go beyond the supported end date if not playing ironman.
- The degree to which a trade node is monopolized (as determined by the greatest power share) now adds up to 100% to the privateer power modifier.
- Province tooltips now show more information regarding blockading fleets.
- Blockades are now indicated on the map with special border graphics.
- Implemented a new system called "States & Territories", where states gives most benefits of being non-overseas, while territories have autonomy and is considered to be overseas for many rules.
- Added Corruption, which is impacted by being behind in tech, being a big nation, and can be combated by investment in the budget. Corruption impact minimum autonomy in a country, its ability to do espionage and all power costs.
- Added "corrupt" cheat to set the level of corruption.
A list of the dev diaries for Mare Nostrum can be found here.