Upgrade them at every chance. Build a bigger, better inflatable landing system to handle more weight.
Put a RTG power source on the thing so it can stand up longer and not be so weather sensitive and have LOTS more power to do cool stuff.
Set out a call for competitive designs to existing qa/qc and performance specs, along with weight and dimensions, and let all the kids building robots today flail away at it.
Each rover could have either upgraded and/or new sensor or exploration tools.
It would be appropriate to use the pent up hunger for real space exploration to really kick Mars butt. The spinoffs in advanced robotics would be immense.
While we are at it, put a few more satellites in Mars orbit so we could have a martian version of TDRS(sp?), with selected sites getting full time coverage either form earth or overhead.
The satellites could have new sensors to compliment the ones already overhead.
Ed