A crewed mission to Mars must address a number of complications (especially prolonged radiation exposure and low gravity). It would be the longest and farthest journey in human history. While there are many skeptics and obstacles, the buzz about Mars raises the stakes…and story possibilities.
Martian colonies are certainly not a new concept in speculative fiction (i.e. The Red Mars Trilogy by Kim Stanley Robinson). As we get closer to the possibility of a Mars colony you may find new inspiration. Perhaps your story is an allegory for the European colonization of the New World or is focused on the economics behind settlement. Would a human settlement lead to the extinction of native microbial life? What if the terraforming of Mars also meant a strain on its environment as heavy industry did on Earth?