I actually started writing up a a new constitution a while ago as a sort of thought experiment. It's not finished yet but some of the highlights thus far include:
A unicameral congress, with uncapped membership
A right to privacy, free education, internet, and government transparency
Freedom from religion clauses
Constitutionally limited intellectual property: Copyright is 15 years for corporations, life for individuals.
Uncapped supreme court, 2 appointments per presidential term