IAN W:
But the major unexploited advantage is that you can be fully 'established' 'stabilized' on a GLS procedure with turns and changes of vertical profile as part of the automated final approach. Yes it will make it different for the procedure designers used to using straight lines at fixed descent rates for final approach and the definition of obstacle clearance surfaces may have to change, but the prize is standard instrument approaches to all runway ends anywhere.
RNP AR already offers a stabilized approach with RF legs as necessary, and a short, straight final segment (just prior to DA if necessary). But, both the FAA and ICAO are hard-over on the benign obstacle clearance environment in the visual segment. The equipment requirements are also burdensome.
RNP AR blending with GLS would be the ideal solution. But, that would require FMS redesigns and so forth.
In any case the likes of BHM Runway 18 are unlikely to benefit from future improvements in avionics. BHM 18 actually works quite nicely today with an advisory VPATH and the PAPI requirement.