Spawn / Lair Level:

I think it should be possible to associate named, abitary, numeric values with lairs that can be used to determine how powerful of creatures it should pump, the chance of spawning something on a given turn, and how powerful the guards will be when someone comes to take the lair out (this is in addition to game modifiers like creature spawn rating). Then there would be a tags for defining the calculations spawn chance, spawn rating, and encounter rating. There would be a list for each ecounters and spawns, and in each you would define a group of creatures to defend / spawn. Each item in would be associated with a rating at which it shows up. If more than one is defined for a given rating, one is randomly selected. If there are none at the current rating level, then it would go to the item(s) defined for the closest rating that is less than the current value. If the rating is bellow all the levels for spawns / encounters, then there would be none.

I'd like to see 'clearable' monster lairs. 

The lairs would spawn monsters until you physically go in and clear them out. Using whatever 'spawning' formula is appropriate.

Finding them would probably involve moving into the lair square.  You'd get a dialogue stating you'd found the lair entrance, and it would ask you if you'd like to enter...

Even if you chose not to enter the lair at that time, there would be a chance of an immediate encounter, because you've run afoul of the lair guards.

Tackling lairs would be difficult, i.e. you'd need to face a LOT of monsters at once.

Obviously, lairs of the 'lesser' monsters would be easier to clear out than the tougher monsters.

Successfully clearing a lair would 'shut it down' for a number of turns, and upon successful clearing of said lair, you'd be able to raid the lair treasury, perhaps finding an artifact or magic item(s) along with the gildar reward.

New or recently reactivated lairs would become stronger over time.  So, if a recently cleared lair has just become active, that would be the best time to clear it out.  With this in mind, you might want to station a sentry/guard post near a cleared lair to keep an eye out for trouble.

If a particular monster is a 'resource' for your kingdom/empire (say Darklings, Spiders, or Wargs), perhaps you could discuss an 'alliance' with said lair instead of clearing it, allowing you to hire 'mercenary' monsters.

To make things more interesting, any monsters from a lair who successfully sack a nearby city would increase the strength of their own lair, and maybe add some gildar to their horde as well.

If this were implemented, I'd suggest that there would be special lair battle maps, in place of the plethora of battle maps we have now.  A single lair map would be easiest, but perhaps clearing a lair would involve two or three maps? I.e. Lair Entrance, Main Lair, Throne Room/Queen's Chamber.

If multiple maps were involved, a player could 'turn back/retreat off of' any given map and give up on clearing the lair, but of course the potentially weakened lair would still be active.

This would be a reason to introduce some maps with narrow passages, changing the dynamic of the tactical battles a bit.

Also, Gwenio1, I am in agreement with you, that you shouldn't know just how well defended a lair might be until you enter it...

Excellent ideas.  Add my vote.

And, only sovereigns, their champions and summoned creatures should be able to enter the lair.

 Something I posted:

    Elemental has a wealth of unique monsters and it determined to waste them. the four monsters per empire is a wasteful system, instead monsters should come from rare tiles like spider lairs that spawn randomly. Everything from rock spiders too ogres should be in the game, though monsters would be devided into neutral good and evil, with  only the corresponding empire able to summon good and evil. Also each "lair" would have mutiple levels of a monster like a rock spiderling lair that lets you summon spiderlings and giant spiders. You would be limited to what you could buy though by the level of a tech research with 4 levels, replacing the 4 researches now implemented. for example an ogre lair would require lv 2 research to create young ogres and lv4 to create ogres. While darklings, spiderlngs and wolves would require lv1. Lastly, remove the option to create a random monster as a prestige bonus, its a useless attempt to allow players to have fun with monsters.

Add the two together so you need to conquer the monster spawning lair

As I stated, it *can* be done, but it is not possible to have them randomly placed and each and every one must be created with events.

I'd go with a singular map. It would be far more important to adjust the monster placement and AI. Rather make it a bigger map than a more complicated system...
If you run the generic AI, all monsters all across the map would start charging the intruding player. That's pretty bland and doesn't make for a good dungeon crawl.

To make such a map interesting in any way, LOS rules would be required.  Kinda silly if you'd see all monsters on the entire level when entering...

The next step would be "assist rules" for the monsters. A classic "aggro chain" from the MMO of your choice. That allows you to ambush a monster unit in a dark alley of the dungeon without alerting their full force.
Some guard / patrol units could have a "shout" ability to alert monsters in a larger radius, adding more gemeplay elements such as backstabbing, stunning or silence spells.

Assaulting the throne room would not be as easy because the "boss" and some guards would be rather close together.

Without LOS and assist rules, the entire concept would just not be worth the effort.

Also a consideration: with the entire dungeon being a tactical battle, the map cannot be too large. Imagine moving all your units at 1-2 squares at a time. On a large map that would be a nightmare and anything but fun.

Good ideas for encounters, though lairs should use the same mechanics as battles during a quest; therefore it is special encounters in general that need work. After all, at its heart a lair is a quest location that spawns monsters near it till the "quest" is completed (typically by clearing the lair out). Beyond that the only things that might go into the creation of a lair that a quest might not is ways to scale up the encounter / reward (if any) as the lair grows stronger (caused by global monster level increased, special events like having a spawned stack raiding a town, and/or having been active long enough). Of course, lairs should not be quests regardless of how similar they are. After all, they should not require research to clear / find.

One other thing that lairs should have availible when desigining them is ability to control where they appear, such as making yeti lairs appear in artic terrain and not deserts. Others might be more likely (or required) to appear on certain resources, such as a rift through which fire elementals emerge would be most likely to be found on a fire shard (or only found there). This way they will not have illogical placement. This might also be good for some quests, though most of those should be able to be placed anywhere. Edit: to keep the terrian from changing near the lair, there could be an 'anti-ZoC' that block ZoC from covering the land in a certain radius, prevents players from building near it of resources, and makes the land unchangable by magic.

For the spawned monsters, it would be good to be able to give them a behavior (such as having them wander near the spawn point or go far and attack towns), if the game ever supports such things.