I've been collecting ideas for building a Mars Rover. This can easily cost a few thousand dollars so I'm not quite sure I'm willing to fund such a venture, but it's fun to dream (and plan).
Apparently lots of people have been inspired by the NASA Mars rovers, as there's lots of projects out on the Web. NASA has even created an "open source" robot project, with downloadable plans.
So far I've been scanning the toys at ServoCity for ideas for the steering mechanism and rocker arms. The scale and weight of the overall robot help decide the size/power of the motors and servos. It's a bit of a balancing act. I also want to, unlike most of the "civilian" designs, somehow tuck my motors inside the wheels or otherwise not have them hanging down near the ground as many/most of the rovers I've seen seem to do. There's a huge impact on ground clearance if your motors are down there banging into rocks and such. The difficulty seems to be locating some suitable wheels. I can't imagine simply 3D printing them, as plastic wheels wouldn't last very long on rocks or concrete. I want a robot that can actually roam outdoors.
The ExoMars (and ExoMy) robot uses a simpler "Triple-Rocker" design.