I have been playing Transport Tycoon for a long time. It's my all-time favorite game. TTDPatch is a very impressive set of patches for the game, adding lots of new features that seem like they would be impossible to implement in a patch, especially by authors who don't have access to the source code. Open Transport Tycoon is an open source port/recreation of the game, and it's what I've been playing lately.
In Transport Tycoon your goal is to build a transportation empire using buses, trucks, trains, airplanes, and ships. The part I find the most fun is laying out tracks, railroad stations, and train signals. I think most of the players spend most of their time on railroads for this reason. I think I would enjoy the airplane and ship parts of the game more if I had some control over layout. All you can build right now is 6 predefined rectangular airports and 1 ship dock.
For several years on the Open Transport Tycoon forums, there's been talk of “modular” airports that would allow players to place hangars, runways, taxiways, and gates to build their own airport. However, airports are implemented with finite state machines that carefully manage multiple airplanes in holding patterns, landing, taking off, taxiing, going to hangars, and stopping at gates. Richk67 has created an impressive patch for custom non-rectangular airports. However his experience has been that the finite state machines are just too complex for players, and his patch is instead aimed at people making addons. Pikka has an alternate proposal, but take a look at the example state machine code for the simple airport. That's way too complex to make players deal with. There have been others who have argued that it should be possible to let players do this, if the GUI was good enough. However, Richk67 actually wrote some code and implemented custom airports, whereas most of the posters are just talking about proposals or ideas, so I generally believe him when he says that the finite state machines are too complex for most players.
My hope however was that there would be a completely different
approach to airports that wouldn't involve the complex finite state
machines. Playing with path based
signals, a new feature
in OpenTTD 0.7.0, has made me more optimistic. With the signals in the
original Transport Tycoon, a train made a decision when it reached a
junction with signals. The third party patches added new kinds of
signals, including allowing complex logic in TTDPatch. Path-based
signals work quite differently. Trains reserve tracks ahead of them
and only proceed if they can make a reservation. Can we apply the same
idea to airports? Instead of following a fixed state machine, we might
be able to reserve space ahead of the airplane (landing flight space,
takeoff flight space, runways, taxiways). I had only just started
thinking about this problem when I came across Dimme's
demo of a
proposed player-created airport system using airplanes reserving space
they want to use. In the demo (a Java app), you can place gate,
runway, taxiway, and other tiles to build an airport, and then run a
simulation to see how the airplanes use it. It's quite impressive!
(Note: I couldn't get the
jar file to run on the Mac so I had to
recompile it myself, but it ran fine.)
Dimme's demo shows that there's a different way to build airports that may not lead to the same complexity as the finite state machines. MarkyParky, who works with airports in real life, has good things to say about Dimme's demo, but also talks about holding patterns, which turn out to be more complicated than I imagined. I don't think that realistic holding patterns are necessary but it's something to think about. In any case, I don't think any of the discussions are going to go anywhere. Richk67's patch is the only code I saw, and as far as I can tell, the main developers have rejected his patch. Dimme demonstrated an alternate approach but there's no proposal for how to make it work within the OpenTTD code.
Dimme's demo is good. It's simple to use, it demonstrates the idea, and it lets you actually try building an airport to see how it works. After playing with it, I decided I wouldn't build an airport building demo. I think my ideas aren't different enough to make it worth it. However, his demo reminded me that I should be building more demos of ideas I want to explore. Coming up with ideas doesn't teach me as much as trying them out.
Update: [2012-08-18] There's a modular airport mod for Cities XL.