I have similar ideas, that makes the retainer system even more worthwhile than AH:
First of all there should be remote interactions with a retainer via pearl. Logically we can't give an item, but we can see what this retainer have and most important, we could manage the markets from everywhere. Lets say we have a high demand item to sell and we have a lot of stacks too. What do you do? Staying near the wards at the current system. With remote, you don't. So Ul'dah isn't be needed. You can craft and idle in Gridania and your items get sold in Ul'dah.
Second idea. Retainer need a licence for market wards. A merchant license is only loan. So if a character quits, the merchant license is over after a month. SE could set the license prices very low, because it's only a way to get rid of old retainers in the wards. It's an very easy way.
Set NPCs at every city, which represents the markes of the other cities. I really like the idea of 3 different markets. It worked very well in FFXI. And i think if the server population grows, characters will be set at other cities again. Why not?
The NPCs only reflect as a representative. Although you can't buy, you know there is this item in Gridania. One step further this NPCs could also buy for you for a fee e.g 15% and send it to you. Must be worthwhile to go to the city yourself.
Because there is no Moogle magic, those representatives could be a replacement of a Send menue. Although i think a home would be much better to send the items. I want to craft my own bed again ^^ and i want a way to send items. I don't need an AH, which seems to be uselees in many points anyways.
The most important part: I can finally buy HQ items and materials . With the old AH FFXI system they were all bought in an instant and tossed buy GRMTs and we were forced to look at the characters outside of Jeuno to buy incredible overpriced items. Because this system need a bit of time, RMTs don't do this as often. I think the retainer system gives more players a fair chance to buy items at reasonable prices.