I'm a big fan of written constitutions, but if we're going to make one, we should start asking other nations what they want in it (i.e. term limits, form of government, etc.) For that, we could create a dedicated sub-forum.
There are also some other issues we should address, like citizenship (including how nations may both acquire and lose it), emergency legislation, individual rights and guarantees, constitutional amendments, etc.
One thing I really liked about your draft is the Declaration of Open Rebellion and how it needs 2/3 of the vote, instead of 50%+1. This 2/3 system should also be implemented for constitutional amendments, in my opinion.
IRL, I'm a law student in a country with a civil law legal system, so I'm used to this sort of codification. If you want, I could help you with the draft.