You dont look at a GBAS standard, you look at the accuracy required for CAT III autoland, and meet or exceed that.
That accuracy level is already in place in the standards, it is only a matter of validating the system to meet the standard.
There are many places in the world, and in the military, where the validation does not need to be decades long as with the FAA, but months.
There are GBAS CAT III autoland aircraft and procedures already in operation, much of it government, but there are others...
Also, in the US, currently GBAS is not funded by the FAA so the airlines have to pay for everything (ground equipment, site installation, flight testing, aircraft avionics upgrades, operations, etc.)
a GBAS system is part of the airport infrastructure, same as an ILS, and other NAVAIDS. The airlines do not pay for the system at the airport, the airlines pay for the system on the aircraft.
To outfit an airport with a GBAS system costs about $2 Million, and is good for 26 runway ends and that cost includes maintenance of the system. This cost also includes the approach procedures.
An ILS system costs $500,000 per runway end, and requires about $100,000 a year in maintenance, per runway end. The ILS has all of the multi-path and interference issues, and GBAS has none of these.
The simple costs of it all show that the GBAS is far less expensive, and far more powerful for operations.
If you were an airline paying for the navaid, and the capability... what would you want to pay for?
EDIT: Per the FAA
"The Federal Aviation Administration (FAA) work program is now focused on validating standards for a GBAS Approach Service Type-D (GAST-D) (CAT-III minima) service. The program currently projects a GAST-D GBAS system can be available in 2018."
"ICAO should approve Standards and Recommended Practices for GBAS Category II/III (GAST-D) systems in 2015."
Under SESAR JU, we had GBAS CAT II/III standards approved and validated back in December 2015...
There are a few States which have already approved GBAS for CAT III, and procedures have been designed and are in use.