Which lets the average person feel more like part of the project: voting in a few polls... or being able to own the game for a week and being able to freely discuss ideas in the project's thread?
It won't become garbled, because there will be guidelines developed in-thread (which can be ignored, but will probably in practice be followed). If someone adds something that doesn't fit, the next person can remove it without permission (it no longer belongs to the previous person, after all). The garble will cancel itself out in the end. Some volunteers may even volunteer solely to help tidy things up rather than add new stuff (I know I may well do this).
There are two sides to this project: editing the game, and discussing the suggested aims/plot/whatever. They can be quite separate, although the latter should incorporate whatever the former does (even if just to say "someone please remove factor X") and be quite dynamic, and the former may well depend on what the latter says (if the volunteer that week wants to stick to the suggestions, which they probably will because liking the suggestions is probably why they volunteered in the first place). It's quite an interesting dynamic. People can participate in either/both sides, whatever they want.