Depth and difficulty of mandatory courses that could be elective
In such an inflexible curriculum, this may not be possible, but one thing that I frequently encountered in my undergraduate and graduate education was courses with an option "add-on" component.  The lectures were the same, but the students who opted into the add-on were expected to do extra work outside of class.  In different classes, this can take on different forms, such as extra assignments, or a final project, or a stronger version of the same project everybody else was doing, or something else entirely.  In exchange, the students a) get a deeper education in the material, and b) receive additional units for their transcript.  The latter can be a significant incentive, particularly since students usually don't opt for extra units unless it's a subject they are likely to receive a good grade in.