Perhaps some techs increasing habitability but for only 1-3 types of planets not all each time. Say something like 'Colony Shelter' for planets with harsh climate (would not apply to tropical or continental planet). But Tropical planets would get a specific habitability boost, etc. It would make colonization much more interesting. Might do the mod myself if nobody comes up after patch 1.2 (no time before summer)

I recommend making the cost of colonization higher instead of making planets less common.