We live on a terrestrial planet, and one on which we are undertaking a grand experiment to see what happens when we dramatically increase the proportion of greenhouse gases in the atmosphere. As a result, I think that furthering our understanding of Venus as a terrestrial planet and a greenhouse atmosphere carried to extremes is a compelling target for exploration in the next decade, and is for me, the target for the second most compelling mission for the coming decade.
The complete presentation by VEXAG on goals and objectives for exploring Venus can be found at http://www.spacepolicyonline.com/pages/images/stories/PSDS_IP1_Smrekar_VEXAG.pdf
Venus has been essentially ignored by NASA spacecraft for over 15 years (brief studies by spacecraft en route to other worlds have been the only exceptions). The Europeans and Japanese, however, have sent orbiters to this world, and the Russians are planning a mission in the coming decade that may include a lander, orbiter, and balloon. I believe that the U.S. should join the party in the coming decade.
The VEXAG analysis group chartered by NASA has put together an ambitious plan for a highly sophisticated Venus Flagship mission. This mission would include a very capable orbiter, two balloon platforms, and two atmospheric probes/landers that would survive for many hours on the surface for detailed soil analysis. Unfortunately, this mission would cost over $3B and requires technology development in several areas. As a result, it is proposed as a mission for the 2020s and not the coming decade.
However, VEXAG members have also proposed a less capable mission, the Venus Climate Flagship, as a possible mission for this coming decade. In the types of mission elements -- an orbiter, a single balloon, and a single lander -- it seems much like the full Venus Flagship proposal with the duplicate platforms removed. However, the Climate Flagship would focus on using existing technology, resulting in less capable measurements but doing them as much as a decade sooner. For example, the full Flagship mission would have brought samples into the landers for analyses that would take several hours to complete. This would result in expensive sample handling mechanisms, an air lock, and the requirement to survive on the surface for almost a full Earth day. The Climate Flagship proposal, on the other hand, would use lasers to illuminate or melt the surface materials with the results analyzed via spectrometry through a porthole in less than an hour. Similarly, the full Flagship proposal would have carried a radar on the orbiter that would have mapped the surface with resolutions as fine 5 m. The Climate Flagship would map the surface at "10X" better resolution than the Magellan mission. This would result in mapping resolutions of several 10s of meters. (The exact figure depends on whether average or best Magellan resolution would be the baseline, and it's likely that the Climate Flagship proposal hasn't been studied in sufficient detail that the final resolution is known.)
There isn't a firm public estimate for the cost of the Climate Flagship. A swag in the presentation describing the mission suggests a figure of ~$1.6B, but notes that your "mileage may vary." However, with international cooperation, the NASA contribution might be substantially less. Several nations are interested in missions to Venus. Russia, for example, wants to fly its Venera-D lander this decade. The Europeans have expressed interest in flying a balloon platform. NASA might contribute an orbiter for data relay from landers and balloons, to remap portions of the surface with radar at higher resolutions, and carry cameras and imaging spectrometers to extend the atmospheric and surface studies of the European Venus Express and Japanese Akatsuki orbiters. The RAVEN radar mission has been proposed for the current Discovery mission selection. NASA also might contribute a lander if the SAGE mission is selected as the next New Frontiers mission.
The Decadal Survey has three flavors of landers and a "climate mission" (no details on what that might include) on its list of candidate missions it is considering. If any of these missions are recommended, or if the SAGE lander is selected, NASA could participate a series of missions that would perform the science of the Climate Flagship. (I think it is unlikely that the Decadal Survey would recommend the entire Climate Flagship mission, which as its cost become better understood, might be substantially more expensive than the swag quoted above.) The Survey could recommend a single element -- the orbiter or a lander -- or a combination such as an orbiter and atmospheric probe. It could also recommend no NASA-led mission, and instead recommend participation in the missions of others. The Russians, Europeans, and Japanese could supply missions that meet the goals of the Climate Flagship. China and India are also on the verge of being able to send probes to other planets and might also participate.
Except for the recent European and Japanese orbiters, our knowledge of Venus is based on missions with decades old technology. The science questions motivating a return to Venus seem compelling to me. I believe that NASA should make continued exploration -- preferably as part of an international effort -- a priority for the coming decade.