I would like to see an external-webinterface, where you can open the marketboard, buy and sell stuff and check your retainers. A smartphone app would be nice, too.

In my opinion it should be a website accessed from the lodestone by chosing your char. Then you have basically the marketboard search interface, your inventory and gil plus a dropdown-menue for your retainers. Here you can do all the stuff you can do when you are logged in, even verntures.

You could also implement a search-engine for your inventory and your retainers like on the marketboard (implementing this in the game would be useful).