Underfire:
If DEP does not require criteria, what would be the foundation of the design? (especially OEI?)
All engines operating uses either TERPS or PANS-OPS departure criteria. (or, the operator can submit its own all engines operating departure procedure for FAA approval.) OEI is a different matter, it becomes the domain of the operator's performance and planning department. In the U.S. that means adherence to 14 CFR 121.189 as at a
minimum.
Most operators use AC 120-91 for area containment, but the use of the OEI RNP tracks, such as used as Cairns, would be quite acceptable for OEI planning and performance compliance. It would, however, require a certified RNP AR airplane and an RNP AR qualified flight crew.