I think that the ability to create custom buildings would allow players to experiment wth different gameplay styles to see if their ideas would balance or unbalance the game.
If this feature is already in the modding pack, let me know. The only feature I've seen so far is the ability to replace textures, which is still cool.
Also, it would be a very nice feature to be able to design your own planet with custom buildings. Although it wouldn't be an online feature, it would allow players to test their ideas to the fullest extent before presenting them to bast. I understand that this would probably be a lot of work for bast and I don't want to complicate things for him. I just think that this feature should be added in the long run, no matter how far in the future it has to wait.