I was thinking about a party I had long ago in Kuftal Tunnel, we were always aware that Guivre could spawn and wipe us out. It very rarely happened but it was still kind of cool and added a sense of danger to know it could happen. He could spawn and had a wide patrol path, near wandering around the whole zone.
There was a similar kind of feel in Konschat, where a Tremor Ram or other could randomly be wandering around and the first you knew was the shaking and loud noises.
I think one of the problems with modern FFXI is the lack of danger in zones, you can still be attacked by aggro monsters and this makes the world feel more real but you're never really in danger as you used to be.
What do you think about adding a singular really high level monster to some older zones that just wanders around in a really big route and that is aggro to players? You could call it a beastman gladiator or something and make it level 130 or more. It could be WoTG type model with all the armor on.
The point would not be to kill it since it would have no drops, but just the added sense of danger to know it could be around or could spawn. Maybe give it a title for those that wanted to kill it if they had good enough gear and backup.