First change the borders of the haven world. At a certain distance from the center, land must end. Rather than be a sharp cutoff, make it so only water will spawn this distance away and land can touch this point but will gradually ebb away, making natural looking islands.
After the land cutoff point, water. The ocean gets deeper and deeper, dealing more damage to ships until it would do enough damage to destroy a q1000 knarr in 1 tick. If you have a q1001 knarr, congrats, you can see the usual black edge of the world at the cost of a shitload of tarsticks, you're a champ.
^That would be cool on its own, a more "lore friendly" or "realistic" border.
24 hours prior to a world reset, if possible, Jorbtar could announce it. Usual Ragnarok debauchery occurs, but now something's different. The world border expands, land becomes shallow water, Shallow water become deep, deep becomes ocean. This destroys everything on the land, including hearthfires, forcing players to log on in the wilderness.
To prevent new player confusion, you could change the log in/character page during this event, with a little "what is Raganrok?" or "What's going on?" link that just pops up a little text box explaining what's up.
But why? The landmass shrinks until all that's left is one screens worth of space, perhaps only for the last hour. From there - nothing; there's no reason to fight. The last surviving hearthlings could throw a tea party... But we all know how it would go down. You could go there early in the world and build a maze or some kind of prison for fun. You could block it off entirely so no one can play for the last hour lol.
It would be interesting to see how people react to this, and could enable players to do some wacky, creative shit. Maybe we will see civilisations compete for the territory at the center? Maybe we will see a faction build an "Apocalypse City" Every world at the center. Maybe they would welcome the peaceful but turn warriors away? Maybe they're hog the final landmass for themselves? Maybe Jorb and Loftar could end the world with the land turning into deep water and watching the last hearthlings drown for their sick pleasure?
EDIT: I totally neglected to consider the underground in this post. But you know what? The water destroys caves and mineholes, if you're underground when that happens, you stay underground. Smash your hearthfire and respawn in the wilderness, or skulk in the darkness like a coward. You can lore it up as the land drying and the new hearthling emerge from the soil, degraded and weakened.