I'm thinking of ideas for improvements/techs/etc. that would potentially allow a player to play "tall" on a fairly large map. Not saying any of it would really be optimal, but it might be interesting. These ideas should try to be "moddable".
For example, I just successfully tested an improvement that costs 1 admin and increased planet class by 3. So it is better than a normal terraforming tile... it could be a great improvement in and of itself... but it is limiting the size of your infrastructure.