-Firstly, to make vehicles effective, hauling needs to be overhauled. Large/Heavy objects should tire dwarves out, especially when being carried up and down stairs.
--Vehicles/animals have low, medium, high and maximum carrying capacity, the last reserved for larger vehicles like wagons. All have higher capacity than dwarves. (low capacity allows a single dwarf to be carried in addition to the operator)
-Animals should be trainable as pack animals. They have light carrying capacity, but aren't fatigued by goods. They should however require training, a pack-saddle and somebody to lead them. Food may also be needed. Pack animals can be harnessed to certain vehicles, and also to winches providing power to machinery.
- Dwarf/Animal/Mill-Powered elevators and cranes should be almost required to haul heavy goods long distances vertically. If poorly constructed, they should wear out and have a chance of breaking, for extra FUN value.
WHEELED VEHICLES
- Wheeled vehicles drastically lower fatigue (when dwarf-powered) over roads and smoothed terrain, but cannot climb stairs.
- Small Vehicles are constructed from wood at the Cartmaker's Workshop, a 5x5 building, or from metal and adamantine where appropriate at the Forge. Large vehicles such as wagons are built from parts constructed at the Cartmaker's. Weight is included in fatigue-reduction calculations so it's better to build your wheelbarrows out of Feather-tree than lead.
-- Wheelbarrows: Dwarf-powered; low-capacity - good for hauling bins and short-haul mining and construction
-- Carts: Require a pack-animal; Cannot be made from metal; medium-capacity; chews up vegetation when not on a road - good for large-scale logging and constructions.
-- Mine-Carts: Dwarf or Animal-powered; High-Capacity; require tracks (can be built into elevators) - good for large-scale mining and hauling tasks. May allow a complex power system.
- Wagons are built from parts at the Cartmaker's and assembled on-site. They have maximum carrying capacity, but require two pack animals and are extra-large. Useful for doing things outside of the fortress and selling to caravans.
BOATS
Boats would be useful in fortress mode now that we have cavernous lakes.
Canals, barges and locks should definitely be included. Canals were historically a very, very effective way of moving large amounts of heavy stuff around. A barge, a rope, an animal and a channel with floor space next to it makes for an excellent way of moving things around one level. Forts could also construct water-based elevators to lift bargeloads of heavy goods long distances, with the suitable FUN risks.
- Constructed at the 5x5 Boatbuilder's Workshop, boats can only be made from wood and adamantine, and must be launched from a ramp or jetty. Being launched from a natural ramp will increase wear on the boat, eventually rendering it useless. Boats are operated with the "Boating" labour and skill, but do not fatigue dwarves.
-- Rowboats: Dwarf-powered; low capacity; move freely across water.
-- Barges: Animal-powered; high capacity; moves along designated canals and must be next to the shore.
-- Ferries: Dwarf, animal or Machine-Powered; Maximum capacity; Moves along a single route between two buildings, which are linked by lengths of rope or chain.
SHIPS
-Adventurers should have a risk of being press-ganged in port cities at night, along with the ability to lead a mutiny, be marooned on an island, etc. Adventurers should also be able to captain or crew a ship.
--Crewing a ship may not be interesting enough, and if included should probably be mostly abstracted, with the captain providing tasks at intervals - fighting a boarding action on an enemy ship, foraging for food, trading in the town? Captaining a ship should definitely be an option however.
--Ships should require regular maintenance, including scraping for barnacles (leading to keel-hauling or time in a drydock).
-Given the trade goods on offer at least as far as 40d, humans seem to be the most nautical race in the DF world. As such most trade-ships should be from human civilisations - though player forts at the very least should be able to buck the trend and build dwarven ships. Elves and merfolk could possibly trade using whales and other sea-life as pack animals.
-Beach forts should be able to construct docks for trading ships; ships should carry large amounts of cargo, but perhaps let sailors out for shore leave and press-gang random citizens just before departure.
-River forts should be able to construct piers for trading barges, if there is another settlement along the river.
- Ship construction in Fortress Mode should be a large enterprise with large potential rewards.