I am by no means a programmer, so take my points with a grain of salt. I have however worked with World Machine Pro and Terragen 3, as well as 3dsmax and Modo. So I think I know a little bit about procedural generation from a purely practical point of view (of an artist using it).
I think the greatest challenge of making procedural planets is the Terran planet. Or, any planet with more than one climate across the surface. With a desert planet, you can use cosine and boolean functions, and some fillet functions, to create various dunes (Parabolic dunes, Star dunes, Rolling dunes, Transverse dunes, etc). And some mesa rock formations of course… but for a Terran planet, you would have to combine all the forms of ice, desert, foliage, terrain / rock formations such as tundra, fjords, mesa, mountain peaks, mountain chains, and ALL the forms of dunes etc…
And when you go into detail in any of these facets, there is even more inside of them that you could go into detail with.
With ice, you can have polar ice caps, ice sheets on mountains, regular snow, frozen lakes, etc. That’s just ice.
What I’m trying to say is that For a Terran planet, you will need to have nested procedurals.
1.First a terrain procedural that handles where there are plains, tundra, fjords, mountains, ice sheets, etc.
2.Then a base coloring / texturing procedural that handles the distribution of these various climates based on some algorithms and rules.These could be tied with longtitude, the density of the atmosphere and the radiation of the sun.
3.Then you go into each climate and have a procedural for each of them…
Ice, Desert, Green, etc
If we take Earth as inspiration, just inside our own “green” spaces, we have a huge variety from the giant Californian red wood (Endor in Star Wars), to regular pine trees in Northern hemisphere, to birch, to ferns, to yew, to oak, to fruit trees, to jungles with palm trees, to rain forests that are in certain times of the year completely flooded. Cacti in arid places…
My point is that nesting procedurals into each other is going to be super difficult to do in a game engine. I would like to hear someone like Flavien describe how he sees this being handled in a realistic way, without overburdening the computers.