The problem most people have with vehicles is theme. They look at minecraft and say that it's medieval, or else a fantasy realm. I respectfully disagree with that. I assign no stereotypical theme to minecraft, but rather what I think will most improve the feel of the game, it's "flavor" if you will.
That said, I'm not looking to see spaceships and jet airplanes and free-wheeling cars. Thus I will present in this thread a range of vehicle options, coupled with a proposed description and implementation. They are ordered by environment, and within each classification, the vehicles are ordered by how acceptable they might be to the community (most to least).
To start making vehicles, one needs to create a vehicle crafting square. To do so, a new block - the vehicle crafting block let's say - needs to be crafted nine times. The recipe should therefore require advanced materials, but be high-yield. These nine blocks are placed in a 3x3 square in the desired location; this square becomes the vehicle crafting square. Right clicking anywhere on the square opens the crafting GUI. The grid in the GUI should be large - about 5x5 or 6x6, maybe more. One major difference in vehicle crafting is that sometimes more than one piece of a certain resource is required in each spot. This keeps vehicles from becoming to easy to obtain.
Now, the last stipulation is tenuous. If my Mechanical System suggestion were to be implemented (or anything comparable) there should be another requirement. The square needs 2 mechanical energy inputs to work. If you have no idea what I'm talking about, don't worry about it. If not that, then a bucket of lava might be a necessary input.
Once the player has input their materials into the grid, they click a button on the GUI to assemble the vehicle. The vehicle is then baked like a furnace smelts ore, using a slowly filling arrow. The time is different for each arrow, If there is enough space and all requirements have been met when it is finished, the vehicle spawns on the square. If there isn't enough space, the GUI tells you. That's about it. Now for the vehicles themselves.
Water Based VehiclesObviously there is already one, the boat. The boat remains as is except for a major speed reduction.
The obvious next step after the boat is a sailing ship. The sailing ship would be about four blocks high and 8 long, submerged about half-way. The surface of the ship would be solid (the player collides with it) - actually all the vehicles in this suggestion are that way. Thus, players can stand on the deck of the ship while at anchor (not really at anchor, just stationary) or control it via a right-clickable wheel at the aft end. The ship would be fast (current boat top speed), moderately durable, and able to hold some cargo.
The next ship would be an iron-clad steam-boat. It would be slower than the sailing ship, faster than the (new) boat, and a lot more durable than either. It would hold only as much cargo as the sailing ship, and be about as large, but riding lower in the water. One possibility is that a cannon could be mounted on its foredeck.
The final and least likely craft is a submarine. It would be small, slow and allow underwater excursions with unlimited air.
How To: To make water-craft with the system, it would be necessary to construct a drydock, with the crafting square at the bottom. Once the vehicle has spawned, you would let the water in to sail it away.
Rail Based VehiclesRail-based vehicles also have a precedent, the minecart, which will remain completely unchanged.
The evolution of the rail vehicle is a bit different than boats.
The first addition is obviously the train. Trains will be a modular system, with the ability to link cars together. Instead of riding on one track, like the minecart, trains require two in parallel (it may be necessary to make tracks a bit cheaper). The first train-car that must be crafted is a locomotive. The locomotive unlinked has a top speed twice that of the minecart. Each additional car subtracts from that. Linking additional locomotives negates speed reduction effects. A player would place coal in engine then enter and set the throttle to desired speed. You could stay in the locomotive or get out and let it move away.
There are many possibilities for auxiliary train cars. Obvious cars include mass storage cars (stacked on a flatbed), a passenger car (mass transport in SMP), or just a flatbed car for show (or it could be a moving platform to build on). More dubious cars could have mounted weapons like bassilae or cannons, or perhaps even edit-able billboards (for SMP or otherwise). There's also the automated mining application. A drill car is pushed in front, followed by a track layer car, a cargo car (which could conceivably auto-capture resources), and the locomotive. It would drill a tunnel and collect the ores. There are more possibilities, but I'll leave it at that.
The final rail-based vehicle would be the wide-base chassis. It travels using two rails, with one block space between them, as opposed to the train. It requires powered two minecarts or a locomotive to move it. I haven't really developed this one, but there are two options. The first is that it acts like the aforementioned flatbed, a moving platform to build things on. The other option I will discuss in another section.
How To: To make rail-based vehicles is even easier than water craft. The player need only connect the neccesary rails to the crafting square. The square itself acts like an omni-directional rail surface. The rail vehicles will spawn oriented towards wherever you connected the rails.
EDIT: To move trains around, something like a rail elevator could be built, using one of many suggestions on elevators and the like. The switching system should also be improved.
Air Based VehiclesAir-based vehicles have no precedent. They are a subject of much contention. Obviously, they're included on the list.
The first would be a simple hot air balloon, small, and slow. It would simply be a boat connected by ropes to an overhead balloon. The boat part would contain an engine and have a small propeller. It would move slower than walking pace and be extremely fragile.
The next in sequence would be an airship (or zeppelin, or blimp, or whatever you want to call it). In any case, it would be large, the biggest vehicle yet. It would consist of a smallish cabin under an ovoid (well, a bunch of rectangles stuck together looking like a highly pixelated ovoid) gasbag larger than a sailing ship. The cabin could be physically entered by the player. It would consist of a steering interface, a storage module, and perhaps small twin ballistae, one on each side. Propellers would be mounted on either side of the cabin. The airship would move at 1.5x walking speed. It has moderate durability. Probably would run on a coal engine.
The last air-based vehicle is one I don't expect much support for. It's an archaic prop-plane. The plane would be of very low durability. The plane would only rise very slowly, but accelerate quickly and have a top speed as fast as a locomotive. Therefore a long runway is required to take off. Landing would require a similarly long runway, because the plane loses speed a lot slower than it gains it. The plane would only seat one player, and would have a small storage capacity. Again, a coal engine.
How To: Making the air-based vehicles is easy. They just spawn normally on the crafting square.
Static VehiclesStatic vehicles aren't really vehicles. They're usable machines that don't move on their own, and aren't rail based.
Included could be: A cannon turret or ballista; a crane or an excavation machine; an observatory (I have no idea what use this would be); an arcane teleporter (highly resource intensive); drill platform, drills straight downward in a 1x1 shaft, and only drills a max of 30 40 blocks down Can be carried on a railcar and moved as needed to drill now holes. There may be more. It's a limited category.
With these comes the other use of the previously discussed rail-based wide platform. If you use a vehicle crafting square to make a static vehicle while a rail platform is parked on top, the static vehicle would spawn mounted on the rail platform. Then, it could either act as a mobile static vehicle (oxymoron, I know), or you could move the platform (via rail) to the the spot where you want the static vehicle and destroy the platform. The static vehicle would then drop down and stay in that spot. So, you could have cannons on your wall, or an observatory on your roof. Or you could have a moving excavation machine, a portable teleportation platform, and an armada of rail-based cannons.
So, if you actually read all that...
I'm not going to make a long argument about why these vehicles would benefit gameplay. Suffice it to say, I think these all fit the game. It's a game about mining and development essentially. The logical next step is production. Why not add further rungs on the tech ladder?
As for the implementation aspect, I think it works nicely. It's not too complicated and it's dynamic, able to be used with any type of vehicle.
As to those who will claim it makes the game too easy: Not really, each of these vehicles would require a good sum of resources.
In any case, I see this opening up a lot of possibilities for cooler buildings and facilities. You could have your wharves and drydocks for shipbuilding, linked by rail to your switching yards, a nexus for your material transport system. Hangars would cease to be useless. Underwater bases would be more feasible and more fun.
For best results I would recommend this suggestion in conjunction to my proposed mechanical system. The two would work well together in offering a minecraft with a more steampunk, archaic technology sort of feel. The link is back in the implementation section.
Note: The following is a joke (maybe).
Giga-Drill MachineThe most powerful available vehicle is the giga-drill machine. It is roughly 12 blocks long and 5x5 wide/tall. It consists of a massive, front mounted, diamond embedded drill, followed by a large tracked, heavily armored chassis. The chassis has a retractable cannon mounted on top. The vehicle is nearly indestructible, able to wallow in lava indefinitely, barely scratched by a point blank creeper blast, and able to withstand falls hundreds of blocks long. The player can control speed (max 2x walking) and turning, on the surface, where it can drive up inclines. When switched into drill mode, it moves considerably slower (walking pace), autocollects resources, and the player controls drilling angle, and can rotate it slowly. Switching into cannon mode allows the player to aim the artillery piece while above ground. The drill has builtin storage of about 16 double chests as well as a portable crafting surface, accessible through the drilling interface or from the exterior. The drill can swim through water in drill mode, or drive along the bottom. The air supply is indefinitely long. The drill also features a miniaturized nether shifter. The vehicle can enter the nether at will and exit it at will. However, the single danger to the drill is slowstone. If the drills entire chassis is over slowstone, it will be stuck, forever. The final feature is the drill's dual headlights, a constant source of illumination.
To build the giga-drill, roughly 512 iron, 128 diamonds, 128 redstone dust, 64 light-stone, 32 glass, 16 furnaces, 256 string, 128 gunpowder, and 256 gold are required in the vehicle crafting square. The creation stage for the drill takes about 5 days.