I've been trying to make a start on rewriting the Planets mapscript, though I don't know too much about fractals and python.

Basically, CvMapGeneratorUtil.py uses some kind of fractal system to generate Terrain types, then the PublicMaps/Planets.py mapscript chooses circular areas to preserve as Planets and fills the rest with Space terrain.
One issue with the old 2071 Planets mapscript was that it kept using latitude to compute terrain types, so planets near the "north" or "south" of the map ended up nearly all Tundra, etc. I can't quite figure out all the python details for sure but I think I've now got it working to generate all of the basic terrain types using fractals rather than latitudes, so that terrain types should all be created across the map in a balanced way.
In the new version, the aim will be to enable terrain variation on a per-planet basis rather than a latitude value across the entire map. The basic terrain types (Grass, Plains, Desert, Marsh, Tundra, Coast, Ocean) have variants depending on the planet's
climate type (e.g. for a Volcanic Planet, there will be a chance of replacing standard
Plains with its analagous equivalent of
Batholith).
I'm thinking the best way to do this might be to add a conditional under
if fDistance < iRadius: in Planets.py that changes basic terrains to a variant version depending on the type of the planet being generated.
I've also been carefully working out a system for painstakingly balancing the terrains, planet types, and yields produced. The basic template by which it works is on the
google docs page; it may be a bit too convoluted to explain fully, but suffice it to say I've taken a lot of effort to ensure that each of the terrains and Planet types have relatively subtle variations in yields provided, the overall amounts of which are precisely balanced against all the others. Each Terrain can contain 3 production Yields in addition to Nutrients, and one of these always the basic tier of its yield group which all players start with tech access to. So you'll never get stuck in a situation where you don't have something to produce or research; but there still will be possibilities for research to take advantage of the terrains found in your colony.
Time will tell if it's a stroke of game design genius or more along the lines of something from David Icke's "
Turquoise Period"..

