Grand tour missions and transport routes are such a good incentive to build diverse and unique functional vehicles

[Vl3d]

The thing that gets me most excited about the idea of interlinked colonies and delivery routes is having to build functional vehicles out of necessity. Just imagine having to transport resources or kerbals initially by rover to an sea or airport, then by boat or airplane to a space port from which you launch them to the orbital colony, send them to your gas giant interstellar space port and sent them to another star. That sounds incredibly cool - mainly because you have to design so many diverse and efficient vehicles and actually use them to set up a  route - carrying massive amounts of cargo as efficiently as possible.

But even more than that.. just imagine a mission a-la Ad Astra in which you have to deliver a person-of-interest (maybe a scientist) on a grand tour or to a far away secluded place. Imagine that the bigger mission is split into smaller ones that requires you to build specialized vehicles which transport the kerbal.. let's say first to a close-to Kerbol colony, then inside the Mohole then to the top of the Eve mountain, then to the deepest place in Kerbin's oceans, then to sample Dres' rings, then on Laythe or Tylo to some secluded place, then inside one of Eeloo's canyons, then back to Jool for a trip to another star system.

You would have to either already have specialized vehicles and tourist transport infrastructure available on all celestial bodies, or you would have to build it. In my mind all this sounds very exciting - mainly because you have reasons to build every time of vehicle you can imagine and adapt to each route and environment - both for routine transports and also for one-shot missions.