The fact of the matter is that pretty much everything mentioned in the OP will be addressed at some point and the current market wards system needs not be changed very much at all.
Fact is that while it's true the wards would need to grow and accommodate more retainers in the future it's also pointless to address that now. Why? Well because the game is if anything losing players and the problems for why it's losing players should be addressed first. Then once that is fixed and players start to come back into the game they can up the amount of usage the wards can handle. Doing it now is a wasted resource when the same people could be fixing other issues with the game that are FAR more important.
Increased usability(which pretty much means anything making it easier to find your item such as Key word search, showing +1, 2, 3 and so on and so forth) were already on the list of Yoshi-P's fixes and the reason we have not seen them yet is that the wards are more then suitable already for the vast majority of players and said players want REAL new content instead of nit picking wards functionality.
Bottom line is a AH in any form will probably never happen in the game(despite Yoshi-P saying it was a possibility) and the reason is that most of the players are happy with what we have now(I mean the wards are faster then the AH in FFXI it takes me less time to pick my item enter the wards and buy it then it does for the list to load in FFXI and then make a few bids before i finally get the item i want) not to mention the fact that regardless of buying and selling retainers are needed(they act as storage after all and even with two of them storage is still a bit of a issue so any new AH system that would eliminate the wards(and thereby eliminate them) is ever gonna happen.
The wards work. Any real problem with them is really nothing but nitpicking. They work for the needed purpose and can be changed and modified as needed instead of getting rid of them. It's not like they are gonna stay as they are now forever. They will be added to they will be tweaked and they will be made better. But I highly doubt they will ever be done away with for any other system no matter what it is.(If that does happen I will gladly eat my word but until then this is my stance)