I'd like to witness some kind of sea giant wading to shore, or maybe giants sailing, like in Time Bandits, or the Thomas Covenant chronicles.
The Fomorians of Irish myth were monstrous rulers over both the seas, and the dead, so there might be megabeasts that have the power to make drowned undead rise to attack your Fortress.
Also, how about river gods, like the old Greek versions? They were divine, but not really in the same league as Zeus and the Olympians, or the Titans. More powerful than say a dryad, though.
And, were you to kill the god of the river, the river would then dry up.
For AGD: Killing a dryad might kill her tree, in the same vein.
Sea Hags: Half witch, half octopus.
Immense chains could be forged and then used to span rivers, blocking ships trying to sail upstream. These chains were truly enormous, and each link might be a separate task.
HFS in ocean trenches is a ***GREAT*** idea.
You could have everything down there, from colossal monsters, to whole cities.
Also, swarms of piranha.
New resources: such as icebergs, pearls, ambergris, sea salt, driftwood, meerschaum, whale bone, amber...Barbarossa, could we please get a New Resources section to the OP?
With random monsters coming in, how about random critter corpses washing up onshore? Ofcourse, they might be undead. Random critters might also just show up from the sea, ala Godzilla/Cloverfield/C'thulhu.
It would be nice to have an Easter Egg where a bottle washed up on shore with a message inside.
Speaking of messages, how about a real treasure map? Ofcourse, it might not actually lead to treasure, but it would be great if you found a treasure map in one game, and then founded a Fortress to look for the treasure in your next game. The map might be in a bottle, in a sunken wreak, or in the hands of a pirate.
Diving bells would work fine, I think, in combination with improved swimming. One diving bell probably wouldn't do a whole lot, but when you imagine *a series of diving bells*, it starts to become more attractive. Plus, pressure is as much of a problem as breathing, but dwarfs might be resistant to the effects, because of their natural hardiness and dense bodies.
Speaking of diving bells, how about the existence of some enormous sea-snail, or something, with a room-sized shell?
Since rice was mentioned, how about seaweed?
Also, dwarfmade oyster beds/farms.
Raw fish should be edible, if eaten immediately, atleast once it's been processed. Maybe make fresh fish edible for the first day, and then after that require smoking/salting/etc.?
Overfishing really shouldn't ever become a problem in the game. The only reason we have overfishing on Earth is because of hundreds of years of rampant industrialized overfishing, in combination with industrial pollution. Before the 1800's, they practically couldn't give fresh fish and seafood away, because there was so much of an abundance.
Considering the presence of animalmen (fishmen seem likely), unnatural extinction or severe overhunting of any major group of animals seems much less likely than on Earth, for that matter.
Speaking of which: I'd like to see coralmen defending the coral. A bit like a stone golem, only with jagged, toxic skin.