This is the best idea I've seen here. SCARCITY BREEDS CONFLICT.
Land (and the resources on or under it, or it's sacred value) is the cause of most of the RL World's conflicts. However, you would need to increase the value of land to each nation. You could do this by increasing the population bonus for land and make improvements take up land, so that there is a cap on your inprovements and wonders until you get some more land. To further flesh out this idea, you have to think about new players adding to the strain on the limited resource. I would suggest that Planet Bob could have a natural growth rate of Land (sounds stupid, but think of it as new discovories due to exploration etc).
OR
A much simpler idea could be: make land massively expensive, increase the population bonus (or add some new value/positive benefit) and make the gains of land from war much larger.