Dante and Tick Tock,
thank you very much for your feedback - most helpful :-) I shall address the points using Dante's numbers.
1. Traffic
Following the reports of excess traffic, I looked carefully into the code, and found a bug in the code that I had written long ago to delete excess city cars after they were generated. Tick Tock was correct that a deletion mechanism had been designed, but it had not worked because of the bug. So, from the next version (3.12) onwards, this issue should be fixed. My (brief) tests have shown greatly reduced traffic as a result.
2. Aircraft
I always intended that, as in real life, aircraft should have a much longer loading time than other forms of transport. The loading time should be taken into account as part of the travelling time. The loading time can be set in the pakset, so part of the difficulty is the lack of Simutrans-Experimental compatible paksets.
However, what I have done for version 3.12 is to vary the default loading times for vehicles depending on their type. From now on, 'buses and trams will continue to have a default loading time of 2,000; trains, monorail trains and maglev trains will have a default loading time of 4,000; ships will have a default loading time of 20,000; and aircraft of 30,000. This will apply both to Simutrans-Standard paksets and Simutrans-Experimental paksets made with the next version of makeobj (which will be released alongside 3.12) in which the loading time is not specified manually. This should provide a realistic counterbalance to the high speed of aircraft (in reality, people do not often take aircraft for journeys where land transport takes less than about three hours precisely because of the loading time).
Incidentally, please note that, unlike in Simutrans-Standard (where the feature is optional in any case), passengers and goods pay for the full route distance, not just for the as the crow flies distance. This is subject only to the limitation that, for each leg of the journey, the total chargeable distance should not exceed a certain proportion of the as the crow flies distance. This would not, therefore, impede your revenue for aircraft passengers who take a long way around to avoid a slower land journey. Note also that speed is the only thing that passengers (and goods) measure when deciding which route to take: they do not take into account cost. It would be extremely difficult to design a weighted routing algorithm with more than one type of inter-nodal cost, the relative weight of each is context-specific.
As to the suggestion of having first class passengers - that idea (or, more broadly, the idea of having different types of passengers with different preferences) has been discussed before. It would certainly add a layer of interest and depth, but would be enormously complicated to program and balance. Since Simutrans-Experimental is currently in a long-term feature freeze, that is not likely to be possible for some time. However, it is a possibility to bear in mind for the long-term (and would encompass far more than aircraft travel).
3. Preferred lines and convoys
The difficulty with this, unfortunately, is that it would require the waiting time per line to be stored, which is not currently done. That would add a great deal of coding complexity, memory consumption and CPU time to the computation of waiting times and their uses. I am aware that the system of having a single best line or convoy has drawbacks, but, since changing the system would require fairly substantial re-coding, any change to the system will not be able to take place during the feature freeze. However, a number of versions ago, to ameliorate the difficulties caused by this in some cases, I made the time before which passengers switched from taking only their preferred line or convoy to being prepared to use any line or convoy shorter the shorter that the journey time for their next leg was.
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Thank you both very much for your feedback (and, in effect, bug report) - it is most helpful. Sorry that it is not possible to resolve all of the issues fully at this stage: I hope that what I have done so far and what will be in 3.12 will be sufficient to make the game workable for the time being. Very glad that you have both taken such an interest in Simutrans-Experimental, and I shall greatly look forward to any further feedback.