You know, I've been thinking around, its so cool the main fact that you can visit any planet, land on it and take of all the way to home... yea thats the issue, what if I want to stay on a planet? there's nothing to do with it, I could make a base and that would be pointless, and I was thinking this while I was playing an other game when the light hit me, I don't have the time to create such a big mod, but it would fit into KSP thematic and it would be just thrilling and exciting to design a vessel the best you can and create a base, it would have sense, so here's the idea....
First of, the actual "resources mining" mechanic would have to be diversified, lets add iron, rare metals, water, etc... the point of this its to force the player to have multiple facilities distribuyed around the planet or even multiple moons, on this point you could sell resources for money if you somehow land the containers on kerbin, making the contract useful just at the begining with increasing prices on development and high tech things.
Next step, factories, biodomes and living modules, this one its easy, in order to build anything you need some sort of factory, to make the factory work you need workers and to have workers you have to feed them, so this would has to have some sort of balance, but this would be your main colony where, here, kerbals has to come from somewhere, in other words you have to bring them each one of them until you have come to the next mechanic because so far its just standar KSP.
Factories need also resources, specially metals, propellant and energy, so here its where the real mechanic comes in place, in order to run a factory first of you have to dock a vehicle into the factory module, then press a "record mission" button where it will automatically undock then you have to fly to a resource mining station, dock, get the resources and back to the factory, once docked the game will record the resources used to make the travel and you can then assign the vessel to "patrol" mission making a virtual version of the vessel (not in game but simulated as a route, similar to rts traders to save memory) and each time it makes some travel it will add and reduce the resources from the stock on the factory module, this would automate the resources gattering avoiding getting the player bored with repetitive missions, then if you want more vessels on the same route you just have to create and assign more of the same vessel to that particular route, this way you could stablish a global market network, where you can also automate the travel to sell this things.
And finally, the space port module, once you have factories, you can send the parts to the space port where you can build your rockets anywhere you like, even in orbit if you somehow managed to get the parts in there.
With this you could exponentially do anything you would like, like simulate space tourism, create a growing colony, simulate cities on kerbin (since it would make sence that this routing system would have been already stablished on kerbin from the first day), and many other things....
Just thinking this would fill all the void and make some really interesting game mechanics,... hope you like some of this