NASA announced today that it's planning to send another Curiosity-class rover to Mars by 2020. The Curiosity rover has been a substantial PR bono for the agency at a time when it needs all the good attention it can get, so I understand their thinking. I'm not sure, however, if this is the best course of action for science. Mars has been studied intensively: Curiosity, Spirit, Opportunity, Phoenix Mars Lander, MRO, Mars Odyssey, and Mars Express have all studied the red planet since 2000. Uranus and Neptune, on the other hand, have never been closely studied.
Curiosity cost in the neighborhood of USD 2.5 billion. For the same cost, we could more or less afford to send a Cassini-class mission to one of these outer planets. Cassini, which has been orbiting Saturn since 2004, is NASA's last planned flagship mission to the outer planets. It's been spectacularly successful and revealed many unexpected things about Saturn, its rings, and its moons. Another option would be to follow up on some of these discoveries by sending a pair of missions to interesting moons of Saturn and Jupiter. Europa, Titan, and Enceladus all seem more likely homes for current life than does Mars.
When New Horizons flies by Pluto in 2015, it will mark the end of new spacecraft in the outer solar system. I think that's a shame, and would rather see that exploration continue before another mission to Mars.