Merging the military upgrade
I think the upgrade technology of the demolition ship must be merged with the other military warship upgrade. With this we can re-put demolition ship for civilization with a naval UU.
More resource type on water
One problem of water is the lack of resource. First we had fishing ship who gather food. Then fishing ship who gather gold (oyster whales). Ad beaver hut as a gold resource in river.
For the other resource we can ad for wood : drifted wood, there is new object in the next DLC make them harvestable. Also we ca allow fishing ship to collect wood on water tree already in game (reed, mangrove tree).
A stone income gain be obtain by harvesting corral.
Make shipwreck harvestable with a decay rate.
Deposit resource on ship
Some tiny island are not worth to build a building to deposit ressource here, even the tiny island can be to little to construct any building. ad the option to drop of resource in transport ship.
Aggressive animal on water
In AoE3 there is treasure guardian in water : hippopotamus and crocodile. Making aggressive animal in water who attack only fishing boat. Fishing boat can gain an attack against them (and only against them, not against any military or enemy building)
Make transport unit by more ship
In the Viking sagas DLC a short video show viking landing on a beach from transport ship and there is longship with them, why they didn’t land from the longship instead ? Make another ship carry troop with less garrison space than a transport ship. Also some military technology can increase the garrison capacity of non transport ship by 1.
Bonus if you garrison unit in ship
Also ranged unit can increase the number of arrow of a longship, melee unit can increase the attack of n hulk, etc. You gain more damage but also you loose all your garrisoned units if the ship is sink. Another bonus can be an increase of speed in rowing ship.
Make villager able to build ship
Some unit can already build transport ship in AoE4. Make a third panel in the building menuy of the villager. Also villager can increase the training time if they are garrisoned in dock or if they repair dock with their hammer (they don’t repair, they build ship, it’s the same animation)
Building on water
There is building on water dock, sea wall, sea gate, sea tower. Make fishing ship able to construct those building. No building can be construct in deep water.
Building bride can also be good, also bridge where ship can go below.
More terrain type on water
Water is just water, no obstacle, just flat water without elevation. In AoE 4 they make an equivalent of forest to ambush : Sargassum seaweed and this is horrendous, we can hide in forest but not in Sargassum seaweed because we are above water.
Make terrain who slow down unit like Sargassum seaweed. Terrain who damage ship like reef or iceberg. Imagine an iceberg who can move on the map and damage ship if they come next.
Tempest terrain can also damage ship or lower their range, line of sight.
Fog : A second filter on map can reduce the range and line of sight of the unit (can also function for land units).
Corral can increase the gathering rate of fish trap, god idea to put here your fishtrap insteaf of next of your dock.
Three good videos by admiral Wololo