Market Districts are acting in a similar way to that of spaceports, terminals & shuttle ports in SWG in so far as they create a hub / bottle neck for players to have a greater chance of interaction. The ability to grow player owned cities and get to build functional structures like shuttle-pads brought with them the same player interaction opportunities for player built cities in SWG.
In relation to DU, apart from it not having the ability to have an automated in game shuttle service, though with Lua it is arguably possible. I believe the game already provides an organisation the ability to build there own shuttle pads and shuttle craft etc. Perhaps however there could be, at some time in the future an additional element added to the game that could allow two or more groups of territories deemed large enough to be cities, the ability to create an automated transport service to each other. Not necessarily owned by the same player organisation.
I do like the idea of some sort of attainment system related to organisational territorial ownership which provides levels of progression and there own item / element unlocks or mini-game mechanics. Geographically one based on traditional village, town, city etc would seem good. Additionally in the future a similar politically linked attainment system within player owned organisations may add another aspect to the game.