I'm very interested in a coding challenge, whether its making external tools or a in game mods. I like all the ideas in the thread so far, especially the noncombat focused gamemode one. I'd be willing to donate too once I know the challenge.
Heres a suggestion off the top of my head, you should probably not make it part of the first challenge since it requires people to stick around:
You could have one challenge with multiple rounds. Each round would build off scripts in the previous one with the idea being that you'll make something interesting using all the previous scripts in the last round So say, we want to make a movie studio mod. Each round would potentially be making different types of bot actions. Another would be making a gui to control the bots. The final round would be putting the gui and bots together, maybe adding save/loading support to the scripts.