I've already gotten a terraforming semi-mod working (still in testing), the key is not to modify the 'planetary value' in the improvement at all, but to edit the actual planet that is generated (although it would be a good idea to modify the planetary improvement so that it changes the planetary value correctly). It's not completely ready, but I'll share what I've learned so far for anyone to tinker...
The game has three values that determines how 'habitable' a planet can be, one of which is the 'class', the number that we see. There are also 2 hidden values as well, elevation range and enhancement potential, both of which is done at galaxy creation.
The class by itself doesn't do anything beside determining the absolute maximum number of people that can be on a planet (a class 5 will have ~4.4 Billion, no matter how many farms you build on it). To prove this, get the '20% planet quality bonus' trait in the custom racial picks, you'll find that while all your planet class get 20% better, you get zero, yes zero, more tiles for builting (except for your homeworld, which does get two more tiles), all you get is more maximum population, which doesn't really matter unless you're talking about a class 3-7 planet, since max pop sky rockets past class 7. While this is probably a bug of some sorts (8 points for 2 tiles on the homeworld is a little... lame?), it proves that the 'class' doesn't actually have anything to do with how many tiles the planet can have. In fact, i've played games (mostly after picking neutral ethics), where I have class 20+ planets with 7 tiles (probably a bug as well).
Instead, the two hidden values determined at universe creation does this. The file you need is "GalCiv2\Data\PlanetDescriptions.xml", in it you will see a block of something like this (I've replaced all '<' and '>' with '{' and '}' because of HTML tags):
{PlanetDescription}
{Name}OceanPlanets1{/Name}
{ColorScheme}OceanPlanet{/ColorScheme}
{UsableElevationRange}0.5,0.99{/UsableElevationRange}
{EnhancementPotential}40{/EnhancementPotential}
{ClassRange}7,11{/ClassRange}
{/PlanetDescription}
Your values will differ, because I've already altered mine. The 3 parts that you need to modify is 'UsableElevationRange' , 'EnchancementPotential' and 'ClassRange'.
ClassRange determines the classes for the planets that is generated, aside from determining max population, this also determines the initial value of how many tiles you can use. Don't like those tiny class 3 planets, just change the minimum to 5. Similarly, you can increase the max for that type as well. Here, I've added 2 to both my min and max for Oceanic worlds (and all the others as well, I like the space). In my case, all the class 4 worlds is now class 6.
UsuableElevationRange is a bit hard to explain, so bear with me. The way the game generate the surface of a planet is through a .raw file. Basically, it's a gray scale that determines the height value of all the areas of a given planet. Then, the game picks a planet type ("OceanicPlanet", in the example above), that gray scale is colored according to a specification in "TerrainColorSchemes.xml". This, is one of those hidden values that determines whether a certain area can be improved. What we are concerned about is how much of these areas will be useable, but modifying the color schemes for that is a bit complicated. The easier way is to actually adjust the two values for UsuableElevationRange here. To increase the the number of tiles that you can terraform, you just increase the range. This is done by lowering the first number, and raising the second number. In my tests so far, I lower/raise by .1 (the min is 0 and max is 1), but in the Oceanic case above, the max was already at .99, so I lowered the first value by .2 instead. Basically expanding the habitable areas of the world by 20% (if height was evenly distributed that is).
The last is pretty simple... EnhancementPotential is exactly as it sounds. I'm not sure EXACTLY how it works, but it's in relation to the above. Basically it's a value that determines how much of that extra habitable height, but not originally habitable (determined by the starting class) could be upgraded. Without any basis for this, I went through and just doubled all the EnhancementPotential values and it turns out pretty well, so far...
After doing that for all the existing planet types, I went through and added 2 to all the custom homeworlds and their respective counterpart (because I added 2 to all the classes), edited soil/habitat/terraforming to double planetary value (just to be 'complete'). Save, start new game, and:
Homeworlds (12 class now) has 4 extra spaces per soil/habitat/terraform. Planets that starts out as class 8 has ridiculous potential for terraforming. The only thing I noted is that class 5 and below still get shafted. Maybe it's the type of planet, I'm not sure.. I need more experiments, but I'm too busy playing. It feels so good to have a bunch of space enough for all those silly trade goods and super projects that usually fills up my project list because I can't spare the tiles. On the downside, losing a planet is such a bummer now, lol. Playing on rare habitable planet galaxies is sooo much better.
"The other civilization leaders said I was daft to build a castle on a swamp, but I built it all the same, just to show 'em. It sank into the swamp. So, I built a second one. That sank into the swamp. So, I built a third one. That burned down, fell over, then sank into the swamp, but the fourth one...stayed up! So now you can do the same! Good luck to ye!"
They said it couldn't be done, so I did it. At first, I failed miserably... the planetary value mod was a bust. But I tried again, but my custom homeworld had little effect besides those specific planets. Then I hit the jackpot with planet creation. Now, I'm not going to say I understand how everything works, but hey it works... so far! So now you can do it too. If you wanna...
PS: for those if you who still have no clue after this long post (or your head blew up half way), if you want, I'll realease it as a mod when 1.1 comes out, by then I'll have enough time to balance all the factors and play test it enough to refine it and stomp out any possible bugs (haven't found any so far, but who knows). Plus I'll have to rebalance the cost of the terraforming techs a bit to make up for how powerful they now are.
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I've been playing with that for more than 2 weeks now. All that left is to find the planetary special bonus tiles. But it embedded in the .exe and associated .dlls. 