Also, it would be good to have some extra variety of gravwells. I was very surprised that this wasn't implemented with the second expansion, as it wouldn't seem to take much effort and it might add to the game greatly.
I had a quick look at the Sins Plus extra planet list, and the type I'd like to have in the game would be Toxic planets. As it is you can colonise any planet with two levels of civilian technology and then that's it. Though I wouldn't have the extractor bonuses, having a planet type that it took higher tiers of tech research to colonise might add to the game. Presumably shielding would be vital to colonising a Toxic planet.
Also, there is a Toxic planet in our own system, Venus. Every other planet in the Solar System can be represented in the game- Mars is more or less a desert and Mercury little more than a dead asteroid- but Venus doesn't fit readily into any category.
More specials might also be an improvement!