Much of this blog entry will be editorial in nature.
Mars has been blessed with a wealth of missions over the last decade plus. It has gone from a world in which there were serious arguments about whether there had been geologically significant amounts of water to one where a multiplicity of locations have been found that may contain biotic or pre-biotic chemical signatures. Michael Meyer, NASA's lead scientist for its Mars Exploration Program, had a presentation at the MEPAG meeting on the wealth of discoveries that have lead to fundamental new understandings of Mars. I won't try to summarize them -- his presentation speaks for itself and is fairly short. This is the fruit of a program of on-going missions to study new facets and new locations. And yet we still have just scratched the surface (literally) of Mars.
Another presentation from the meeting looks at possible responses to the results of the Mars Science Laboratory. (The same logic would also apply to results from ESA's ExoMars mission.) If MSL and/or ExoMars fails to find conditions that might have been suitable for past life, then the goal for follow on missions would be to explore new sites. On the other hand, should either mission find signs of past or present life, then the logical follow on mission would be to return to the same site with better instruments or better tools to collect samples (for example, from a greater depth in the soil).
I feel that our two most sophsiticated missions to Mars -- MSL and ExoMars -- may fundamentally change our goals for exploration. For that reason, I personally feel that planning for a Mars sample return (MSR) mission now is too early. I feel that we need more wheels on the ground at more sites before we know the right place to return samples from. This question, explore more or move to a sample return at the earliest possible time, has been debated at length in the scientific community. As a whole, the scientific community has voted to move forward with MSR at the earliest opportunity (which currently would be in the 2020s).
The results at Mars from a focused program show the benefits of an on-going, in-depth program of exploration. I think we will see some of the same benefits from the number of lunar missions in the recent past, on-going, and planned for the future. I personally would like to see the Jovian system made an international focus for the 2020s. By then, the Juno mission will have studied the deep interior of Jupiter. NASA's Jupiter Europa mission will enhance our understanding of Jupiter and its moons, and mapped Europa in detail. If ESA selects the Jupiter Ganymede orbiter in its next mission selection process, then Ganymede and Callisto will receive in-depth study. Even with these three missions, there are more that could be flown by other nations -- simple landers for the moons, magnetosphere explorers, an Io observer, or a Jupiter meterological observer.
Clearly, there are many other locations in the solar sysem that would benefit from focused studies. My opinion is just that, and I hope that you will share your preferences in your comments.