Funny you should mention Stars! I'm pretty sure you could use terraforming as a weapon in that game. You could terraform an enemy's planet from space making it more to your liking and more uncomfortable for them to live in.
I'm a little bored with 4X games having a finite linear tech tree. There's no reason why a tech tree can't be infinite, or linear for that manner. For instance, you could always add +1 or more to various attributes of whatever. Have a look at how research is done for Capitialism or Caitialism II. Research is infinite! It woud be great to adapt a system like
It never has to completely level out in the end; each race's uniqueness could always keep them superior. If every race could, for instance, research up to +30 for a planet attribute by the end of the tech tree, it will still leave the race that started at +20 with +50, while races that started with 0 would only end up with +30. I'm against the idea of a linear tech tree that has an end BTW. I'm all for
You still have the problem of conquering planets you can't use in Galciv 2 because the other race's technology is more advanced than you when they colonised it. I don't think the argument is valid and having different races use different types of planets in the beginning would make the game more interesting. There will still be common planets that everyone wants for one reason or another and as techs are researched, the playing field is leveled out
As regards to planet colonization, I have an idea which, hopefully wil be implemented in GalCiv 3. Each race would have attributes for each type of planet - terran, barren etc. Though research, you could increase your race's attributes - for instance, +3 to barren and so on. The planets themselves would have a type (terran, barren, toxic etc) and a basic class number which could be anything from (say) -30 to +10. The way a race sees a planet, the basic