I doubt there will ever be a monster that's on a timer as long as the kings in FFXI. And I doubt there will ever be a monster that takes as long to fight as PW and AV originally did. The trend of FFXIV is going toward casual access for everybody, and content like the mobs you mentioned pretty much fly in the face of that.

I'm all for them, but I don't think they'll ever happen. They did show Behemoth in the new concept art the other day, but I doubt he'll be on a long spawn timer.