The proposal should describe your topic and point me towards the data you will be modeling. I will aim to make sure your proposal is at the right level of difficulty and suggest what ML features and discrete math concepts throughout the semester will be useful to you in this project. It is likely that some proposals will need some modification or further development, so submitting a proposal earlier—even weeks earlier—is not a bad idea.
September 29 should be the absolute latest you submit your proposal. I can provide some assistance in finding a project topic and "matchmaking" partners, but I don't want people coming to me on Sept 28 and saying "I don't have a partner or a topic."
Can't change a rubric once you've started using it.