I'd rather they fix the major stuff before spending time on any AH functions, never mind the fact that if they're gonna change from Market Wards they should go with the Aion-style Trade Broker, not the FFXI AH...

Anyway, just make Market Wards universal and that will solve a lot of it. Also, making a global chat as others have mentioned would help immensely. One of the major issues is already being handled by SE, and that's travel to the other towns. Because airships allow you to go to all three major towns and are fast (have the instant option), people will actually visit the other towns more frequently. If they did the other two fixes suggested, it would greatly repopulate the other areas, IMO.