As we seem to be talking (partialy at least) about infrastructure, I'd like to show you how I imagine this would work in game mechanics (and what relevant techs would do with it). I hope my idea stays withing bounds of current game mechanics, and if not, it could be trimmed to it.
1. Each planet has 'infrastructure levels'. Think of each i-level as the semi wonder. Planet can (and must) have only one i-level at the same time (unless we go with regions for planets, perhaps based on size?).
2. I-level shows how developed a colony is. Bascially it represents the colony infrastructure in the urban planning usage sense
- cities, roads, industries, population (partially, at least), etc.
Here is a very short and simple i-level list (I hope our will be much more developed):
* colony settlement (created by colony ship): a settlement constructed out of colony ship, providing room for first colonists and some tools and facilities for early growth'
* early colony: a developed colony settlement which has grown into a small town, connected with several settlements in nearby regions
* boomtown colony: where was one a small town, now a city booms. Nearby settlements have grown into towns, and smaller settlements now span hundreds of miles from orginal colony landing site
* medium colony: Colony world capital is now a major city and the entire colony has achieves economic size of an average country. About tenths of the planet is well developed, and majority of planet is inhabited, albeit mostly by small settlements.
* developed colony Colony world has one major metropolis, dozens of big cities and hundreds of smaller ones. It is now similar to a powerful country, or small world empire. More then half of planet area is well developed and entire planet is inhabited.
* advanced colony Main city on the planet is a megalopolis, rivaling majority of homworld orginal cities. The entire planet, with developed industries and orginal cuture now resembles a well-developed world orginal colonists arrived from.
Sure the names and descriptions can use much improvements, but they serve to illustrate the basic idea, I think?
3. I-level requires in order to be build on planet:
* an investment of production points/minerals to build (and perhaps money as well if we are using it)
* minimum population on planet (either in total numbers or %)
* a given tech
(obviously this is an important part for us here)
4. I-levels :
* produce minerals, pp points, research and other points if we use it, money
* determine maximum population size of the planet
* can be 'specialized' - i.e. in industry (more pp), mines (more minerals), research (more rp), others (intelligence, ship construction, various stuff we invent later)
* their efficiency may be dependent on total population in place - as population increases, so does the planet/i-level production
5. Now that we have a basic understanding on what i-levels are, back to our tech discussion.
Starting empire should have a few basic i-levels, or perhaps only one - for its HW. It would need to reasarch some colony doctrines/theories to even develop an early colony one, then more to be able to make the colony growth. Some pararell tech lines should explore 'specialized' colonies, allowing player to create agricutlure planets, military planets, etc. Other, later, would deal with 'developed sci-fi ones' - imagine 'ocean colonization', 'subterrean expansion', 'orbital habitats', 'continent-domes', etc. And of course other techs could make existing i-levels cheaper or better or sth.
The advantage of i-level idea is - IMHO - that it puts an important part of what we defined as construction under one common frame. I hope you like it