Plenty of people are working on possible solutions to further explore Mars. The abiding problem is how to get some range out of the next generation of probes. It looks like the smart money is on some sort of aircraft, and the most plausible idea is to my mind a balloon. It sounds a great idea to send a fixed winged aircraft of some kind, but that's a lot of technology and a lot to go wrong. Physorg has an interesting article on one possible contender for a balloon mission.