I think a PvP add-on that has as much content and changes in it similar to abyssea would be awesome. A handful of objective based modes and zones specifically designed for them would probably get a lot of people interested. The only thing I can think off that holds this back is FFXI's pacing in all of its systems. menus, npcs, everything is a bit too sluggish for PVP in my opinion. Some of this could be fixed to benefit all of FFXI though like having NPC menus display more than 3 options and staying above 3 levels deep.
I haven't done ballista or brenner in years not because I don't want to but because I can't. There simply isn't anybody I know of who wants to play it. This is probably since there haven't been any encouraging updates or additions to it in years.