Well, this is the system I have reached to:
This campaign has a really big group, 12 players +1 cohort +2 animal companions + 1 familiar (familiar doesn't count as separate as I suppose the wizard will be hugging it a lot while she runs away)
On their scape route there will be sections in which they may run without problem, others with lots of "rubble" in which they will only have the possibility of double moves, and others in which they must be more careful or take some sort of skill check to advance (low DC balance for tilted floors and slippery areas, climb for partially blocked passages or jump for small gaps on the floor)
They are almost at the top of an ice fortress of 8-10 levels of height, 20' difference of height between them.
Each round, 1d4 characters are endangered, first of all, the player must roll a listen and a spot check DC 15.
Then, roll 1d8+ modifiers (see bellow):
1: Light debris falls, Reflex DC 12, damage 1d6
2: Moderate debris falls, Reflex DC 13, damage 2d6
3: Serious debris falls, Reflex DC 14, damage 3d6
4: Boulder falls, Reflex DC 15, damage 4d6
5: Slab falls, Reflex DC 15, damage 4d6 + pinned if failed, 2d6 if successful. Pinned characters must make DC 20 Strength or Escape Artist check each round to escape but are immune to falling debris. Free characters may make or assist with Strength checks. If the debris is made of ice, fire spells could remove it, with some risk to the pinned character.
6: Minor Crack opens in floor, Reflex DC 15, no effect if successful, if failed getting free is a full round action for that character or another.
7: Slab of wall/column falls as on 5 but it is bigger and endangers also line of 15' in random direction from target square (possibly requiring other characters to save) and blocks passage (Climb DC10 or Jump DC 15 to pass to the other side)
8: Major Crack opens in floor, Reflex DC 18, no effect if successful, if failed as pit trap 20' fall to lower floor
9: Floor section 25' long centered on character severely tilts towards a random direction (Balance DC 15 to remain on his feet and another one with the same DC to move out of there) Floor section falls to the lower level (20') 1d3 rounds later.
10: Floor section 15' long centered on character falls to lower level (20'). Reflex DC 20 or standing jump DC 20 to reach safe "ground".
11+: Roll twice on table ignoring further 11+ results, the second one without modifiers, and apply both results if applicable.
Modifiers to roll on table:
+1 if character is moderately encumbered
+2 if character is heavily encumbered
+1 if character is making a double move
+2 if character is running
+1 if character fails the Listen check
+1 if character fails the Spot check
When a character falls to a lower level, a Tumble check DC 15 makes the first 10' fall deal only subdual damage and a Jump check with same DC makes another 10' fall deal only subdual damage (but only if he voluntarily jumps down).
I have also prepared a couple of special situations like passages that are completely blocked forcing them to find another way, thin walls of ice that they must destroy to pass, a slab of an external wall falling off exposing them to the elements and showing the long fall down, a really big boulder falling bellow them and destroying all floors some squares behind them on its way down and making the surface bellow the last characters tilt down to the pit formed, having gaps of more than a single floor open in front of them (or between them if the group is too spread out)...