Casey, on day 40's "post-stream" in reponse to modding support you mentioned you wanted "people to have to go into the source code" since it is the goal of the project. This made me think that maybe there could be an achievement which actually required to changing the source code and recompiling it to get it.
The idea behind this would be that people that only downloaded the game for the game´s sake and never programmed before wanted to unlock all achievements would have to do it and would see how relatively easy it is to change the source code. I guess it would have to be an easy change like maybe uncommenting something or changing a value, and maybe the place in the code next to it would have some other fun/interesting change that could be made too, so once they tried doing the achievement they could play around with it (like say changing the player´s speed or something).
Definetely not a fleshed out idea, but there might be something there.