The "plan", for a narrative loosely connecting easter eggs, as was originally imagined by me, was that a long, long time ago, a precursor civilization to the Kerbals had lived in the same solar system. This is evidenced by the face on Duna, the SSTV signal, Vallhenge, the Monoliths, etc.

Their home planet, however, was lost due to a severe miscalculation of the parameters of their interstellar travel system (maybe the player could've gotten their hands on the tech, but that's a bit of a lame way to acquire interstellar travel, being forced to jump through a bunch of hoops and calculate a bunch of stuff...), which resulted in the whole planet being placed in an extremely distant orbit around the sun, where it quickly froze and died. The rapidly dwindling population of survivors managed to launch numerous monoliths across the system, in hopes of seeding intelligent life.

Their plan failed. They made Kerbals.

This planet would not be visible in the map view, or discoverable through any telescope. To find it would require the player to locate several SSTV signals (these and the monuments were the equivalent of what flags are today for Kerbals - a "look, we were here") around the system. Each signal would normally contain a complete list of orbital parameters for the home planet, but over millions of years of degradation, only a snippet would remain.

Once the orbital elements of the planet had been found after decoding and analyzing enough signals, its current position could be calculated with reasonable accuracy, and a mission sent out. Once found, the planet would reveal itself as just a bit smaller than Kerbin, covered in frozen cities, abandoned monuments, oceans frozen solid, and an atmosphere long lost to the depths of space. This far out, the sun might even not light up the world too much, leaving it in a perpetual state of twilight. A very, very, very unusual and alien world.

On the subject of the first SSTV signal - it depicts four precursor critters standing by their monument, their home planet, and the symbol of the planet's world government, a universal sign of peace and exploration. A number would have been added later on, as Duna's contribution to the orbital solution.

Of course, this whole "plan" never really left my head, apart from a few brief teasings that I had a plan! I believe this is the first time I've ever really talked about the full extent of the ideas for a sort of narrative behind the easter eggs (although IIRC I may have touched upon it in past forum posts, like the idea of the monoliths failing and creating kerbals instead of proper intelligent life). Maybe one day I'll resurrect this plan, maybe as part of Alternis, or even in a different game.