I like the idea of a reusable interplanetary craft that will not enter atmosphere, but can be used to ferry landers and people between Earth and Moon/Mars. It would either have a fission reactor or huge solar wings (whichever ends up with the better power/mass ratio when the support systems and structure are included), and would use VASIMR engines. It would be to Moon/Mars flight what the Shuttle was originally envisioned to be for LEO flight (originally as in before budget limitations resulted in the decision to use an expendable fuel tank instead of a reusable liquid booster stage). Assuming that NASA actually builds the base at the lunar south pole in the 2020s, the earliest that we will see a craft like this will likely be around 2040.