Increasing OW speed is a good idea. It will act like 'fast forward' without breaking anything in the process (as everyone's speed will be increased). The current increase is quite noticeable on longer journeys and a good change. It will create problems if too accelerated, but the current values suggested seem good.
Trade currents are good for various reasons. This offers the player more choice. He can use the trade current at the benefit of speed, but at the risk of being targeted
due to its use by other vessels. Or he can choose to stay away from the current, which may be safer, but slower. Also relatively easy to implement development wise.
I like the proposed additions to diplomacy. Something I noticed: Currently an alliance is turned directly into a state of war. Should there not be a 'cool down' from alliance to neutral, then to war? Alternatively all nations should be in a 'neutral' state and then pushed to either states of war or alliance via voting. This will create a smoother transition between states. To prevent the problem where nations are all neutral, one could either allow attacks in both neutral and war states, or perhaps provide more voting options so the player can vote for a small shift or a drastic shift depending on what is happening in the game.
I'm not sure if this is covered already, but in my opinion there should be more functionality for the 'day to day' operations of sailing. Currently if the player numbers are low for whatever reason, there is a not a lot to occupy the player. Some 'single player' elements could be introduced to keep it interesting. I can think of many examples and name a few:
Management of crew skill and morale (e.g. food, supplies, activities etc). Training of crew (e.g. gunnery practice). This can be done as an 'activity' in open water, which can be seen only by the player, like fishing is currently implemented, with the upside that it gives something to do during longer journeys. Or it can be implemented as a type of mission. When the player initiates it, a battle instance is created, which can even be entered by other players. This will provide more missionstypes and goals, but not something to do during open water sailing. Gunnery practice can provide a temporary buff for combat, or lead to a permanent levelling of crew skills.
More interaction with buildings, ports and outposts. Appointing and managing labourers, wages and bonuses, which will affect available labour hours. Taverns for hiring crew or other activities? Exploration possibilities? More reasons to visit different ports.
I love the game, appreciate what you've achieved so far and think the end product is going to be great.
Edit: I forgot to mention. Crew skill can be 'abstracted' into numbers before a battle. Crew loss during battle can still work the same. At the end of battle if crew was lost, one can determine which crew (good or bad ones) the captain loses. Perhaps a percentage chance to lose specific crew? One can make it that trained crew are never lost, or have lives like officers (once determined that a crew member was in fact lost).