Many ways to implement this concept.
1. You could have only loot for the lowest lair, and if you flee ... any that enter have to fight through the upper levels again.
2. You get loot for each level, and defeated levels disappear. In this case, if you killed all but the lowest level, and fled .. the strength of spawns would GREATLY increase. (another way of implementing this is that, while spawn strength increased, there was a chance for an upper level to 'regrow' and thus lessen the strength of current spawns)
3. the upper levels (are always there) aka insta-respawn (like option 1)... but you still get loot for each level, but only for the FIRST time that the level is cleared. (this is either really easy or really hard to do depending on how the code is structured ... I think)
In all three cases ... only good, non respawning lairs would have a bottomless pit of evil.
For instance ... a Bear Cave, Wolf Lair, and Butcherman Camp should be a lair that can be spammed and respawned by the "world AI" while only ever having 1 lair.
Spider lairs can have multi-lairs .... with some being easy and others being SUPER HARD like the above examples.
Elemental Shrines should probably have multiple lairs .... (especially for death demon lairs)
Wildlands should definitely have a few pretty scary multi-level lairs.
Not all multi-levels need to have monster spawns on the outside ... but it would be fun if at least a few of them did.
(some could just be creepy creepy caves that you'd be scared you might accidentally wander into and die a horrible death ... speaking about Wildlands with 'random' loot n lairs)
----> more random "Is it loot or is it a lair" could be fun
Point is, this could be done at any level of complexity, including overly simplistic. And that would be fine too ... even having just one Multi-level would be nice.
I want to separate multi-level from Multi-battles ... because with Multi-battles you have no choice.
THE ARENA is a good example of this ... but sadly, you can only enter it once (as its a contest, not a super creepy cave)
-> well, Imagine the Arena where after each battle you could choose to leave ... and you could always come back (and start from battle 1). And it was only destroyed once somone finally won the last battle. That would be closer to what I'm talking about. The arena only gives you one choice to leave, and its so you can get a new Champion instead of continuing on.