It still wouldn't work well because you are still going to overload the retainer/market ward system.
The retainer/market ward system scales very very badly and is incredibly slow and buggy. It still takes a load of time to actually buy and sell things, and in wards that are overcrowded, and prone to crashing.
A broker system wouldn't work because the foundation would crash and burn too often and can't keep up with the expected volume.
It's like running the NYSE on your dell laptop with windows on it. It just won't work the moment people actually buy and sell.
The retainer/market ward system is pure BS. It's built around the idea of small scale trading. No matter the bandaid, it'll buckle mightily fast the moment it gets into the big times.
Ironically with the subscription base of current FF14 on its last legs, we may never need to see the big picture.
The better idea would be to have both at the same time, a global AH with high tax rates, and a market ward with low tax rates or something to that effect.
Trying to mix retainers/market wards is a step into the destruction path, because it's graphically, physically, and server-wize highly demanding.
If an efficient age of trading like WoW that is so keen on efficiency it's even able to program custom filter, you would have thousands of trades an hour.
You'll get a stock market crash every time the wards crash, or someone recalls a retainer to do some inventorying.