Drunkugly

Combine the colonization mods with some kind of large scale projects to direct your entire space program towards. Space stations that are as complicated as the surface colonies but then are able to support large projects, like a space based telescope that generates more science the further it is from Kerbin. A massive laser for launching solar sails, with the goal of getting them to a significant percentage of the speed of light and going interstellar. Or a large magnetic field generator placed in the Duna/sun Lagrange point to thicken the atmosphere and start the terraforming of Duna into a second Kerbin. Maybe orbital mirrors to heat the surface. Ground bases that act as science generators and when Duna is warm enough with surface water, transplanting life there and leading to varied biomes to explore. All these things being unlocked in the tech tree and needing a lot of science and resources to build and launch.

Been thinking about this for awhile. The thing that feels missing in the late game is some large scale project that you can direct your space program towards. Once you've explored most of the planets, it starts to feel like repetition. There's not a lot to actually do or see on the planets once you've landed a few times. So I would suggest megastructures and terraforming as end game objectives. The idea starts with base building. Right now you can design and build your own surface bases and space stations. What I"m suggesting is a modular system with pre-set parts that can be mixed and matched. In addition, you'd want prerequisites to parts being attachable to bases and stations. The idea is to create a sort of mini game of it. You could start with basic housing for Kerbals, requiring a powerplant and a food growing module. Once these basic needs are met, you can then add further components like mining, refining, science lab. And maybe one step further, enhancement modules like water recycling to improve the farming module. Each one of these modules would require Kerbals to staff them, thus they'd act as a break on unrestrained building and force you to balance everything. Once you had your base or space station sufficiently developed, it would then be capable of supporting one of the mega projects you can unlock in the tech tree. A space station could support a massive telescope that could take several launches to assemble and place, the idea being the further from Kerbin you are the more science it will ultimately be able to produce. Or a space based massive laser to launch solar sail probes; the idea being to get them to a sufficient percentage of the speed of light to make them interstellar. On Duna you could build a base that would support terraforming it into a second Kerbal. You could add large projects that would thicken the atmosphere and allow liquid water. The few ideas I've seen would be large space mirrors to heat the surface, or a large magnetic field generator in the Duna/Sol Lagrange point to protect the planet and allow the atmosphere to thicken ( https://astrobiology.nasa.gov/news/how-to-give-mars-an-atmosphere-maybe/). Once the atmosphere was thick and warm enough, you'd begin to see lakes forming on Duna. Then you could add modules to your ground base that would transplant life to the surface. Ultimately you'd end up with a second Kerbin, with varied biomes to explore. Lots of things to add to the tech tree and to shoot for. At any rate, I know there a mods that do some of this, but it'd be nice to have it all integrated into the stock game.