Someone once suggested on a space blog that we move Venus to the opposite side of Earth's orbit so that it is exactly Earth's distance, but opposite the sun, then make Mars a moon of Venus, and terraform both.
Wild idea, but would make for a cool sci-fi movie.
It would take a lot of planning so as to not disturb earth's orbit during the move. The most stable station points would be 60 degrees before or after earth's position.