"Unless of course some player stops it from growing."
And this is the one thing that has to be guarded against. If it becomes, Turn 1, go find and terminate all Troll and Spider caves and nests, or else, then what is the point.
I can imagine, with Balance in place of course, that a Troll cave could spawn and grow into a menace, in your region, that may cause your ultimate downfall. While cool once, by the 5th time, you are forced to play either super defensive vs the Troll/Spider onslaught that is coming, or play the Turn 1 scenario every game...
Let them exist and thrive even, but if all they do is grow until they need to be eradicate, then perhaps that is how they should see the human population growth, and it intrusion into "their" region and go all hell bent to eradicate you... Just hope you advance fast enough to keep up. LOL
E:WoM should not ever become E:Kill the Trolls and Spiders or Die.
See, this is why there should be three tiers 1) Boss Lair 2) Early Expansion 3) Late Expansion
The Boss Lair should be nigh indestructible for most opponents, and should express to the player that this is a place where death would follow trying to enter it. Meaning, that its impossible to try and "kill off" each lair because they each have a boss lair that shouldn't even be touched by a 10-foot pole unless with a huge army or very high level Champion squad.
The second phase (the phase most ppl interact with) should be the Early Expansion. This phase should have VERY slow growth, weak opponents, and beaten down fairly easily. This growth should be relatively linear, meaning that no matter how close it is to the next phase, it's growth doesn't grow any larger.
Third Phase would be Late Expansion. This would "usually" only result if there are no nearby competing lairs, and if it has gone Unhindered by Champions for a LONG time (nearly 100 turns or so) then it reaches critical mass and turns into late expansion. This still starts out as very slow growth, but slowly accumulates growth speed on a Logarithmic Function. However, even with faster growth, its borders grow just as slow, if not slower, than early expansion. Why? because now a Layer of "Strong Growth" is growing out from the Boss Lair, and it takes more growth points than weak (early) expansion. This Strong growth designates areas of the Lair at Large which can have stronger minions and Mid-Bosses. Also, if its a buildable lair, it is at this time that the building gets larger, more rooms are added, etc. Or if it is a static building like a Castle or Dungeon, then certain rooms get populated by more powerful enemies.
Also, if the Lair is a static Place like a Castle, then weak growth outside of the castle will be weak armies (a couple mooks) that venture forth to try and attack things. Once the entire Lair/Castle is made of strong growth ... if its Spiders they will save up to send Spider colonists to build another spider nest ... if its sentient beings they will slowly build up a small army to take over another lair. (So basically, if they can make new lairs, later strong growth spreads to make new lairs, however if they only exist in previously build lairs, then they will create a small army to invade another lair, or settlement)