Port vulnerability
Not really a fan of time slots as they might just stop pvp port battles from happening - I'm sure most ppl would rather have an intense battle over 2-3 days with the port being wrestled back every few hours, rather than a port being taken and the defeated force having to wait till the next weekend to get a force available for the allotted time.
Maybe have a system where the port isn't fully taken over until 18/24hrs etc have passed - this will allow the defeated side to counter attack but also allow the devs to make losing a port the disaster it should be for the players with ships/goods in port.
Method of attack - two options
Would rather have a physical system to keep it somewhat realistic - maybe a nation can only attack a port within a certain distance of another national/allied port due to supplies?
Announcements
To keep strategy/cooperation etc important it'd be nice to keep any announcements as short as possible before the battle started - maybe the attacking force needs to blockade a port for 30mins/1 hour 'setup time' before the battle starts. A global msg would only go out when the battle started (or even a few mins before) but it would give an advantage to those nations scouting/communication and working together.
Port battle invites
Personally I don't think there should be any barriers to players joining a port battle if they are in the area and wanting to fight. The balancing act should probably be done on the world map as a whole - if a nation becomes too large future ports captured could cost extra to use/keep friendly. Maybe have a rebellion system where nations with a large number of ports could have them flipped back to a previous owner, or even have a nation have to pay an upkeep with each port captured being exponentially higher, if it wasn't paid there could be a %chance it could flip. Maybe add an option to raid a port for gold/xp bonuses if a force doesn't want to control the port