I would like to propose the idea that we run sprints during the competition. If the competition is 100 days, that's a lot of time, and I think it will be easy to get lost, discouraged, sidetracked, or bored. I think doing sprints will set up a cadence that will help people keep making progress and give people an easy opportunity to get back in the game if they do run into problems.
The basic idea with sprints is that at regular intervals, you release what you've got, and get it in the hands of people who can give you feedback. Sprints also help you focus on working on only the most important stuff at any given moment in time, because you have to show what you've accomplished.
"Release" could be defined as an actual download (possibly a paid download, if you want to go that route). It could also be a demo video instead. Depending on your goals, it may or may not include the source code. It's a public display of what you've accomplished to date.
Sprints can be varying lengths, but shorter is usually considered better. We don't all necessarily need to be on the same cadence, but I think it would help us all if we were.
I think our best options would be either one week sprints or two week sprints. (I like one week sprints, personally, but if I get outvoted on that, I have no problems with two weeks either.) Three or four week sprints starts to seem a bit too long for this kind of competition.
Perhaps we can even take an hour or so at the end of the sprints, do a little Twitch demo of the progress, and/or swap our games around and play each others' games for a few minutes each and share feedback with each other.
I don't want to turn sprints and sprint demos into a huge arduous task. That would take the fun out of it. The intent would be you just package your stuff up in a ZIP file and post it and link to it in the forum, or that you just put together a 3 or 5 minute demo video of what you've done, with no more than maybe 15 or 20 minutes preparation. No need for a ton of video editing or anything like that.
I think sprints would help us to break a gigantic 100 day competition into more bite-sized milestones, where we can see progress at each step of the way and set new goals. It also gives us an easy way to rejoin if one week doesn't go so well. If you're taking a vacation, just skip a sprint. Busy with work? Skip a sprint. Get bored with the game you're working on? Call it "done for now" at the end of a sprint and pick a new game that excites you a little more in the next sprint.