datiswous

Technically I see 2 possible ways.

1. One is where everything takes place inside one mission and you teleport to different enclosed areas of the map to do subquests.

2. Another is a campaign style mission. You start inside the cityhub. When you do a subquest, the mission ends, but the next mission you load the specific quest-map. This questmap you load is depended on what subquest you choose to start inside the map. Afterwards you load the 3th mission in the campaign which will be the cityhub map again, but via persistant info, changes some things inside the map (for example, you can't do the same subquest twice) and you have more gold maybe, etc, etc. So if the cityhub has 5 subquest missions that can be played in any order, the total amount of missions in the campaign would be 10, but the amount of map files 6. Because if you start in the cityhub map, after each subquest mission, you return to the cityhub map again, except for the last mission. When you make missions later you can add them to the cityhub again and make it bigger.
(This seems all possible with changes made in TDM 2.13 , but I'm not 100% sure.

It's at least an interesting project to work on)