A loose network of specialists. Central control is not evidently necessary, and certainly not necessary to any great degree. To the extent there are certain optimal solutions to the problem of getting to Mars, anyone attempting to do so will discover those solutions and cooperate for reasons of efficiency with others who also have found them. Specialists will come and go as need for their expertise arises and ends. For example, the launch vehicle designers busy in year one will not be involved in year 15, nor will the city builders have much to do until then.
Central control adds layers of management, delays implementation, increases costs and waste.
This network is now in being, and it's evolving rapidly. The Mars Society is both node and catalyst, creating other nodes through its chapters and task forces, and by networking persons and groups who'd previously worked in isolation. Red Planet, Inc. runs small research and development projects along similar principles, pulling together diverse resources for specific projects on an as-needed basis, without creating any permanent institution beyond its founding partners. As an aside, this notion has a theological component as well. We're not looking at the problem and devising means to solve it.