The final shape of the course will be, in the end, their doing. They are, therefore, committed to the content, the process and the evaluation of their learning. We deal with any mistakes and their consequences together as a class. The mistakes have proven to be fewer than I might make with my own course creation FOR them and the course highlights far surpass the highlights I might have provided. I have noted more positive peer pressure and peer support rather than the usual teacher-student conflicts and student competition.
Classes seem to have a greater purpose and intimacy.
If no consensus or equanimity can be reached in shaping the course as a class, then it is best to turn to the fall back position of your initial design. It represents what you would have done anyway. In two tries with this student-driven approach, I have not had to fall back on my initial design and I believe the courses have been better than I might offer up on my own. I have used this approach in classes of less than 25 students but this student-driven course design is possible with larger classes by eliciting student input at the first class. You can then adjust the course design, delivery and evaluation to meet those needs and interests.