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I totally get where you're coming from, it would certainly be cool to see what people come up with creating everything literally from scratch. But, that's not how it is and it sounds like you're putting a constraint on yourself that's unnecessary since it's not one of the rules.

I doubt any of these games were created weeks or months in advance aside from the one that was submitted to the last jam that CT hosted, but a general ground rule I've seen is that you can use pre existing art and code for your project, the caveat is that you don't know the theme prior. If your pre work doesn't match the theme, then you need to redo it, so it's to your advantage to wait.