Just as a side-note: I have several plots myself. Only one has no building. And right before clicking the button to build on the last one recently bought from the secondary market, I thought about this.
Ah humans… As soon as they arrive somewhere, they start building and potentially destroying what’s there: nature, culture, elements of the past…
The number of plots without any building is decreasing. Fast. Without any consideration on whether it’s a bad thing for the environment or not, as it happens in many places on Earth.
National parks were one of the best ideas related to nature conservation, I was thinking: maybe plots preserved from any “CLNY production” building can be a different kind of plot, dedicated to the planet Mars itself.
@brother you mentioned this game would certainly have pets in a foreseeable future. Maybe @eugenefinch already has some plans there 
How about making these plots places where unique resources, wildlife and flora can be found and collected?
To access these lands, you’d have to pay the land owner, which would also be responsible for managing the “hunting”/“gathering” quotas in order to preserve what’s there and make nature thrive on his/her plot.
For people who already built: a deconstruction mechanism can be considered, which would cost CLNY tokens as well (half the construction price for instance).
A player is then able to make an important choice early in the game instead of following one unique path: a long term role for his plot. Will it be robots/CLNY production focused or rare resources/wildlife production focused?
More possibilities to make a plot unique, be there for the long term. Beautiful and rare things require patience.
It also slows down the CLNY issuance.
Cheers