With several colonies throughout the systems, some degree of traffic is... expected? Desired? Required? Sure, KSP doesn't simulate all of Kerbin's cities, populations and industry, but those aren't space assets. KSP 2 is going for space colonies and industry, and if you have those, you've gotta have space traffic.
I don't see how it could be fun for the player to run every material and passenger demand in two or more systems, between potentially a dozen colonies. To have no traffic at all seems... sad. The reasonable solution, then, is somewhat obvious, and appears surprisingly simple.
The player could design and launch ships with either cargo, material or passenger capacity and enough DV to make the journey between the two or more desired location. If A and B have refueling facilities, then the ship only has to have the DV to get from A to B at worst system disposition, and vice versa. The ship would be deposited into storage at either A or B, and launched to make the journey by the colony AI. It would automatically cross the distance, refuel, deposit/pick up goods and passengers, and travel back.
What this gives the game is 3 things. First, it gives a sense of life in the system, like you are not the only thing moving out there.
Second, it lets you design various ships for the mission, and demonstrates how real life interplanetary traffic would look and function.
Third, it gives some sense and reason to colony growth. In any reasonable scenario, early colonies would need shipments from more developed locations (unless the colonization effort is truly huge). The player could do that themselves, but we have Elite Dangerous and Eve Online for when we want to play space truck driver.
The big issues I can see:
- Not easy to make: You'd have to make pilot AI that can set up paths between A and B without being insane.
- Atmospheric entry would have to be handled with two ships and an orbital station. The space freighter would deliver to the station, then the station would use shuttles and lifters to move the cargo to and from the surface colony. Lots of mechanics there.
- Resource demanding. If done wrong, a dozen ships moving across the system could be hard on the simulation.