Yep but there's too much space there, finding the other player on a real-sized planet will be quite problematic. You'd have to track him down by listening to radio signals or something. That can be interesting game too but it will take a lot of time as well. I think that in order to employ the whole planet the game has to be either a singleplayer one that can be played over a long time, or it has to be a massively multiplayer and persistent one, where you can encounter others more easily and build up the ties.
The number of players and objects would be high, but they will be also spread over a very large area. If the game events were also paced in something more close to the
actual time, the load on servers doesn't have to be very high.
What I have in mind is combining a single player city-building game with resource mining and FPS elements with a persistent online world, then adding more features like the ability to trade and interconnect with other players, forming alliances and so on.
This could be the terraforming game others wanted to see too, only in a persistent online world. Even if the world didn't allow for real-time interactions between the players initially, i.e. the server would just synchronize what players are building on their allocated land, it could be quite fun, don't you think?
There could be also a simple resource management involved from the start determining what can be build, later extended by adding the trade so one can send trucks or trains for some materials to the neighbors .. and so on