I had thoughts of an investment or bid system but ran into the problem that in larger environmental challenges the characters would have to have a lot of points. I also didn't want to have to introduce a new point system for the characters. There could be a solution to that but it didn't seem evident to me. The thing that bugged me was that, even if successful they characters will have reduced their pool of points for the next challenge. Even if you're okay with that, how does that fatigue effect the rest of their performance? I wanted something that tied into the rest of a system cleanly so you didn't just ford the Amazon river and then carried on with the rest of the game like nothing happened.
Those were the concerns I had. Just thoughts that might be useful.