Unfortunately, in my experience of GURPS this pretty much completely breaks down. Further, even in systems where it usually works, there are downsides. Perhaps the encounters are too dangerous, especially in a densely-inhabited dungeon where they would bring others. If handled poorly they can become a joke or a severe annoyance for the players. Sometimes they send the wrong message, or they simply aren't wanted in your dungeon.
Nevertheless the function of rewarding time and action economy is worthwhile to salvage. I originally thought of using random encounter tables along with faction diagrams to give reinforcements to the set-piece monsters, but the more I mulled this over, the less I liked the idea. One of the reasons random encounters work is because they give immediate consequences to the players. Generally, immediate consequences are better for informing behaviour than mediate. With the reinforcement idea, or any other scheme that puts off the results, players are more likely to shoot themselves in the collective foot. In the example given, they'd most likely keep doing whatever until the monster density became so great there was no feasible way to proceed, or until it maximized - and then there would be no further disincentive.
Then I had a breakthrough: why does a wandering monster system have to involve monsters at all? If the central point is to make the players economize their actions, that can be done without monsters as easily as with. Further, avoiding monsters makes it work for systems like GURPS as well, where the attrition due to combat is nowhere near as strong as it is in D&D.
To that end, I give you a few 'timers' I've been thinking over lately:
The Timed DungeonThe essential feature here is that the dungeon itself has some integral timer which makes exploration progressively more dangerous or difficult. Some specific examples:
- The dungeon is flooding: every <interval> more and more of the dungeon is underwater. To do this you need to know the source, and the relative elevations of various rooms. I'd recommend giving rooms four states - dry, ankle-deep, chest-deep, over-your-head, completely-filled - and rate the source(s) by how many stages it can fill per interval. Generally I'd eyeball the map and flood downhill, with rooms getting to ankle depth before the water spills over to a lower place. Bonus points if you use some fluid other than water - I'm partial to mercury or poison gas.
- The Archmage is out: every turn roll a d6. Once you cumulatively roll 5 1's, he's come back. The party better skedaddle soon, because he's a 60th-level ubermage able to cast Elric's Flaming Haemerrhoids at will. This works well with archmages' towers, elder dragons' dens, demons' lairs, and generally anywhere you can stock a big nasty that the party knows would squish them flat in an outright fight. A variant has the bad guy already there, but temporarily neutralized - asleep, behind a failing barrier, whatever. Season dice and intervals to taste.
- The dungeon is unstable: maybe it's situated in the caldera of a live volcano, or in the rift between the astral and ethereal planes. Or maybe the entrance is an old mine shaft that's under serious stress. This has much the same mechanics as the archmage one above, but after a certain accumulation of rolls the entrance will be closed, or the dungeon will collapse, or whatever. A variant on this is the 'clockwork dungeon' - where the map changes every so often, making navigation difficult or impossible. Maybe the dungeon is a wizard's toy, or it's slipping through time, and staying too long will mean you have to deal with dinosaurs or barbarians where you expected your village to be.
The Timed TreasureWhereas above the dungeon itself was becoming undelvable over time, in these scenarios it's just becoming undesirable to do so, due to disappearing reward.
- Kingdom of the Sidhe: After a certain time passes, all the loot in or from the dungeon will lose all value. The adventurers had better retrieve and spend it beforehand! Keep a timer keyed to turns or hours or whatever. Whenever enough time passes, increment it by one. Don't forget to make sure the players know they need not only to acquire the loot, but get rid of it too! An elven favorite is turning leaves into gold, but fresh basilisk blood or psionic crystals that must be preserved by the local alchemist after being chiseled from the walls are also good ones.
- Explosive treasure: Do you really want to muck around when you have a backpack full of white phosphorous in kerosene? Make the treasure valuable but volatile. It doesn't have to be explosive; it might be an acid, or a powerful djinni bottled in a jar and yearning to get out, or carefully preserved bottles of essence of green slime. Every time the characters do something dangerous, it has a chance of backfiring.
- There goes the neighborhood: The denizens of the dungeon have decided for whatever reason to pack up and leave, bringing their stuff with them. Much like the coming of the archmage in reverse. Roll a d6. After 3 or so 1's, randomly or by fiat pick a faction; they exit the dungeon with all their treasure. To complicate matters, the PCs might be between them and the exit.
- Competition: The old standby; have another party racing the PCs through the dungeon. This takes a care and finesse which is outside my scope. I'd think at the very least you'd have structured tables and a general idea of how the NPCs will progress through the dungeon, but coming up with specifics for running this scenario is left as an exercise for the reader. An exercise he'll hopefully then publish on a blog, so I can steal his ideas.