OK, after some thinking here are a couple of pointers that might help with the development:
The polar regions are lacking the detailed coastline polygons, which I think should be added because the visual difference is striking. I'm wondering if you didn't add them because they're ice and thus constantly change over the years.
The next step I'd take after polygon design would be to start choosing visual textures for textures.png and try them out to see how they look, and how they look together. The main issue here is that the Geoscape palette is quite limited, plus the day/night cycle uses different gradation coloring, so there aren't many choices available there regarding colors.
The lack of color variety will have effects on the realism, if you check an orbital picture of Earth the colors will be too bright and the polygons will stand out, but this can be mitigated by choosing different visual patterns for each texture. And the lack of realism is also due to other factors that are impossible to control, like seasonal changes (winter in the northern hemisphere and summer in the southern one), plus elevation differences are not always visible from space. But here I think that realism needs to take a step back to aesthetics because if the globe doesn't look good visually that is going to be an eye sore for the player.
Once the visual patterns are chosen and assigned to polygons.png, then it will be a matter of assigning those textures to the individual polygons, and you're probably cringing right now because there are thousands of them. One way to reduce the workload is to assign to texture #0 the most used visual pattern, and then manually edit the other textures.
But here you'll also have to take a design decision: either only use the original textures, which will allow your mod to be used easily by anyone by simply replacing the world.dat file; or add new textures, either from the Terrain Pack or whichever you like, giving you more flexibility but making the modders have to adapt to your system.
This decision also applies to the terrains being used: you either stick to the vanilla ones, or you add additional ones from the Terrain Pack or elsewhere to represent the new biomes, etc. However, since the terrains are assigned through the rulesets, modders can easily choose which ones they want to assign to each texture. The important thing to remember here is that the two systems (visual Geoscape textures and terrains used) are used together, but are independent from one another regarding design: one is edited through world.dat, the other through the ruleset. So if world.dat is finished, then someone else can take it and adapt it to their needs regarding terrain assignment.
There are a couple of issues that modders will also have to fix, and I'd advise leaving those fixes to them, because they involve changes in regions.rul: city placements and missionZone definitions (so that UFOs don't land on the ocean/lakes)
Finally, about the terrains used, with the Terrain Pack I've designed it like this:
* There are several biomes: Farm, Forest, Desert, Jungle, Mountain, Polar, Tundra, Taiga, Grassland, Steppe and Savanna
* Each biome can have different terrain versions assigned. This is done by reusing existing sets of mapblocks and giving them a different visual look by assiging different MCD sets
* The existing generic sets of mapblocks are: DESERT.MAP, FOREST.MAP, DESERTMOUNTAIN.MAP, FORESTMOUNTAIN.MAP and FORESTPOLAR.MAP. By using different MCD sets to them, I can create 5 subterrains to most biomes, each using different mapblocks.
* There are a number of unique sets of mapblocks for terrains where I couldn't do the previous trick: the temple versions of Jungle and Desert, the Jungle Mountain mapset, the different version of Farm, etc.
* These terrains are then assigned to each texture, according to their distribution in real Earth, but also trying to keep their appearance balanced
