Terraforming is a neat, but probably complicated idea. If simplified, it probably doesn't required much more of a suspension of 'reality' than everyone having reactionless drives capable of relativistic speeds and conversion of matter to energy.
Is it worth the effort to put together for the game, I don't know. I like to write the stories, not the rules.
As you point out below, procyon, Terraforming is probably only a useful concept if Type T/ST planets are sufficiently rare and if Terraforming was capable of being carried out in a sufficiently short time (even within the game's compressed time scale) at something approaching an affordable cost.
As for body location and habitability, that actually is a lot more mutable than what some might think. If you popped into Sol system through a warp point a sufficient amount of time ago, Mars would have had a magnetosphere, slightly denser atmosphere, and liquid water on it. I don't think there are many people left who will argue whether Mars used to hold liquid water, its just a question of how much and when. Depending on the time frame, Venus might be much closer to habitable than what we currently see. Earth has managed a much longer habitable period, but as you say, location, location, location.
As for Mars and its magnetosphere, you'd have to have showed up about 4 billion years ago, since that's the current estimate for how long ago Mars lost its magsphere. And remember that the solar system is about 4.5 or so billion years old. AND IIRC, Earth's atmosphere is younger than that. As I mentioned earlier, from what I've read on the topic, smaller planets appear to have difficulty retaining magnetospheres due to their small size. Also, smaller planets have weaker gravities and thus a weaker ability to prevent lighter molecules from escaping its gravitic influence.
Regarding Venus, yes, it's possible that that is true for a couple of reasons. First of all, it's believed that liquid water zones migrate outward as stars age and emit more energy. So, it's likely that Venus was closer to the inner edge of the LWZ at some point in the distant past. Secondly, Venus may have had a decent magsphere at some point in its past, though I don't recall reading anything on when it's believed that Venus lost its magsphere. I've also read that it's been hypothosized that Venus may have been hit by one or two moons over the eons and that those collisions are responsible for its very slow rotation.
I agree that there may be some "slop" in the "habitability zone". But a part of the "problem" in Starfire is the relatively simplistic model being used.
A. Orbits are only measured in single LM increments. This prevents planets from existing every so slightly closer to LWZ borders and having slightly better and friendlier black body temperatures that could mean the difference between being uninhabitable and being barely habitable. The use of single LM increments causes each planetary orbit to have some very specific black body temperatures, which are used to produce the various planetary zone borders, and tends to make those borders appear be very "bright lines". However, trying to do the Titius-Bode tables in non-whole numbers is probably nothing but a total horror show, and only realistically do-able on a purely computer model.
B. The Star Types used only represent a single semi-average sub-type within the actual range of spectral classes represented by that star type. That is, the Yellow star type represents Spectral Class G0-G9 stars. However, the purposes of limiting the number of planet-producing star types to a reasonable number, the Yellow star type is represented by the G5 spectral sub-class's planetary zone range values. If I were to used all of the spectral sub-types represented by the White, Yellow, Orange, Red, and Red Dwarf star types, you'd have a 35 different star type rows in the planetary formation zone tables. But on the flip side, you'd have star systems that had a considerably more granularity in differentiation from subtype to subtype. That is, a Class G2 star (Sol is a G2) wouldn't have the same planetary formation zones as a Class G8 star. A G8 would have PFZ ranges closer to the current Orange type than the current White type.
I've actually considered doing this, but it may be a bit too much for some people, though it would create a very realistic feel when you could say that your binary system was a G3/K7 binary, rather than just a Yellow/Orange binary. If all of these sub types were used, on the primary star type table, you'd instead first roll for Spectral Class F, G, K, or M (rather than a star "color"), then you'd roll 1d10 (0 to 9) for the sub type, with values of 0-4 being ignored for Class F stars. The fact of the matter is that I actually constructed the tables for all 35 spectral sub-types from F5 thru M9, so putting them into the rules wouldn't be difficult at all, aside from the fact that the table itself would be rather more sizeable. The rules for using such a table would be no different than the way the PFZ tables currently work... there'd just be about 7 times as many rows ... but a lot more variety.
Tempature could be moderated with liberation of CO2 from most rocks on a planet/moon, is opaque to IR light so it absorbs heat well (as is methane and several other gases), and would in sufficient quantity would allow for plant/bacterial life if temperatures were adequate. I'm sure 'super science' could come up with equally useful compounds that weren't as toxic as high concentration CO2 would be.
Protection from ionizing radiation would be the problem, as would be the soil of most planets without an atmosphere. Radiation isn't going to care about CO2, and sterile plants would be poor at repopulating themselves. Soil exposed to this same radiation will form compounds that are peroxides and would be a great anticeptic in and of themselves. Not impossible to overcome, just tedious and most likely expensive.
Is terraforming worth it in a game where you can just survey and jump to the next system? I don't know... that is a hard one. If T's got rare (devious SM thoughts at work for a future game if this became an option) and terraforming was competitive with multi month colonization transport, maybe. With the current availability of T's, and the ease of looking for more, I just don't see it getting used unless it was cheaper than looking for one, which would only increase the number of habitables and speed of the economic spiral.
Unless T's got rare, I just don't see terraforming (increasing the number of T's) helping the game.
As I said above, I tend to agree regarding terraforming, for the same reasons as you detail.
