While not quite actual rovers, NASA hosts the Robotic Mining Competition every year. [1] The gist of it is they're using a university competition to see lots of prototypes and innovative ideas. They can build a prototype every year or two, but they can see 40-50 ideas and how well they work at RMC.

Phil Metzger, a scientist who was at NASA Swamp Works for many years and is now in academia, has attended the competition every year. He also ended up at the top of Hacker News 4 months ago in an unfortunate situation. [2]

[1] https://www.nasa.gov/offices/education/centers/kennedy/techn...

[2] https://news.ycombinator.com/item?id=16723099