Like many educators, I have spent many late summer days building or revising curriculum. Backwards design is my friend. What do I want students to achieve? How will I know if they get there / achieve what I hope? Then, how can I build a series of experiences to best help them achieve those goals?
But what if the goals are unknown because we want them to be creative or decide for themselves what to do? Well, learning how to innovate is probably not altogether unlike other learning objectives. There are ways to teach skills, even if those skills are more open ended. And end of "unit" assessments ... they can and should be much more open ended. You just plan time and space for that, instead of filling it with a project of the teacher's design.