Also jam are fun but don't give occasion to work on past work, I'd like to make a game jam that have for only theme to continue working in a game made in a previous jam.
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It's funny that you say that! I was looking for exactly that sort of jam myself, and I found this one that's starting a in few days: https://itch.io/jam/finish-it-jam
The only theme is to finish or at least continue work on an ongoing project! I have a Devtober project that has been ignored for months now, so I'm going to use this jam to try and finish it! :)