I am a graphic artist and not a programmer, but I would like to know if there is real interest in a changed economy and if it is worth developing Graphics for it. Crucially, the way new graphics are made also depends on whether developers are found which are wiling to work on such a new economy or not.
Cities
There should be fewer cities, so there is more space for expansion and landscape. A City consists of the city itself and also villages in the surroundings, which are themselves districts, just not adjacent to the city districts themselves. As cities expand, new districts are created (zoning), eventually enveloping the villages into large suburbs. Districts could look something like that.
There should be different kinds of districts, with different properties (cf. next section).
passenger destinations
There are local (travel within city) and intercity passengers. Passengers are created by districts and have districts as their destination. Different districts should produce different kinds of passengers:
* Suburbs produce many passengers to the city centre (and to industrial districts? could industries be located in industrial districts?)
* City Centre produces some intercity traffic to other close-by city centres
* Metropolitan districts produce passengers to other metropolitan, close and far away.
passengers could also be produced to random districts on the map
(this could also be used to simulate tourism, using special relaxation districts eg for cities on tropical islands)
Passengers are produced in groups of a certain size, and then they decide what route they will take on the basis of parameters like speed, frequency of service, avg. waiting time at station, price. (This way 'full load' wouldn't be necessary anymore for getting a good rating.)
possible mechanism
(A) Cities maintain a list of ways to reach other cities ([route #..., get on @ ..., hop off @ ...], [...])
(B) Districts maintain a list of ways to get to other districts of the same city
After being created, a packet of intercity passengers chooses the way to target city on the basis of (A), then on the Basis of (B) they choose how to reach the origin of their intercity route, and on the basis of (B) of the other city, they decide how to get to their ultimate destination after reaching the other city. They then carry with them their journey plan they have to follow. If there is no route from their current district to the origin of their intercity route they could use a taxi. That way, intercity routes could be profitable before building a local network.
Local passengers of course just need (B).
desired effects on game-play
The main effect of this new economy should be focus on network building more than just building yet another new line connecting just 2 stations. It would also open up more possibilities for competitive playing. For example, an established a-b-c-d route (which earns player HANS a lot of money) could suddenly be challenged by a new express route a-d. the number of passengers travelling on each line would then not be determined by the awkward station rating, but by real quality of service.
The new economy should be integrated with a more variable Landscape, like in the following example: part of a river delta, the sea being top left, the river bottom right.

On another place of the same map, an Island:

Now, with the new economy, instead of building like this:

Towns could be linked by buses and maybe a train-line, and there could be one big international airport somewhere in the empty space of the delta, which people from all the surrounding towns access by bus/train.
The Island could initially be served only by boat, but as people get more money/the town grows a bit someone might make the investment of creating an airstrip there. As the whole island could be used (no 4-square catchment area) it would be far more interesting than just building another international airport into the sea in front of it.