I was thinking of a navy battle mod.
It's basically mount and blade except on water.
Instead of capturing towns, the player has to fight to take control of trade points which can be islands or random points on the water.
Instead of the avatar on the overworld being a horse or person, it'll be a boat or ship (duh) depending on how far the character has gone.
Fights will only occur when two enemy ships come in contact, at this point, it will go into the map view, where a fight will ensue as normal, except on ships with planks drawn on to cross. To make it more strategic, there will be access to lower levels of the ship.
As for major trade point capturing battles, it will be more similar to capturing a castle. The battle map: the ship landed on a shore with a castle type building and/or a dock with a part of a town structure.
Capturing trade points will result in automatic deposit of more money. It will also mean more sailors/fighters willing to join you (similar to the original module - the town you capture will mean the village producing more amount of recruits and it's relationship with you increased)
Instead of party size being restricted to leadership quality only, it will also be restricted to the size of the ship you own. If you have the largest ship available and your leadership - morale etc is high enough, you can buy an additional ship that will follow you around (more strictly than leaders in the age of machinery mod). This means that whenever you're in battle, you can have another ship on the other side of the one you're fighting, if you end up having two ships with you, the third one will be connected with planks to one of your ships (because the enemy ship only has two possible boarding sides)
So these are the basic ideas, very similar to the original module, things are just placed in an sea environment.
Ports/Islands = Towns/castles
Ships = Horse
Ships also = Party Size and housing for party
I hope this is a possible idea