Absent constraints, the natural urge of all life is to grow and expand into new ecological niches.
Space Exploration is primarily useful only as a precursor the expansion of humanity (and its children), and those portions of the biosphere we wish to take with us. The guys who have it right are those who look at space as a frontier, and seek to open it to the full powers of human creativity and market forces:
Some of the more exciting near-term visions I have seen are:
Of course, most of the places we want to visit are very long way off. Ultimately, we will want other means of propulsion than just chemical rockets. For a short time, Marc Millis at NASA ran a terrific program at NASA to turn over possible stones looking at this. Sadly, NASA terminated this program, and no-one, not the USAF, the DOE has picked up the slack.
Marc is now continuing to bring attention to this through a new organization.
In the popular media, one place to look for buzz in this area is:
Two wonderful repositories of some of the wonderful designs of the past, when we were a much more ambitious people are:
Another terrific resource is Ron Miller's Book: The Dream Machines
Of course, expansion is going to take a while, and if managed correctly, the Earth should last us until our Sun changes radically, several billion years from now. That creates near term priorities:
1) Protect the Biosphere from catastrophic threats (Comets and Asteroids)
The Earth has been hit before, and it will be hit again.
2) Solve the Energy Problem (With Space Resources)
3) Manage the Climate
Even assuming we can mitigate or solve the greenhouse gas problem, we can expect there to be unwanted changes in our climate, and varriatios in the solar constant. It would be wise to start developing the models and the tools to ensure we can assist the Earth in maintaining homeostasis.