As you’re the evil overlord I’m ok if there’s a mechanic in the game where you’re abusing your own creatures in a ritual/sacrifice to accomplish something, but this must be a special case, not the most common way of getting your army together. This includes getting rid of weak creatures to make room for stronger creatures. Also, if 10 corpses in a crypt give you a powerful creature you should not be able to keep killing rats coming from your portal if that only means fresh ones will enter your dungeon and you’ll get your undead creature for free.
You should primarily kill enemies, not your own creatures.
To accomplish this I'd like to propose three requirements:
1) The player should somehow be punished for killing his own creatures.
This requirement can be met quite simply by it affecting two things: Killing friendly creatures should reduce the happiness of the creatures you currently have, especially of creatures that are close by the place where it happens and those of the same time. And secondly have either overall creature happiness affect the rate of new creatures spawning through the portal, or have self-kills directly affect the creature attraction rate – in particular of the creature type that’s being killed – in a negative way when you do so. Simply put, killing creatures should result in you having fewer creatures.
2) Having low tiered creatures should not negatively affect the number of high tier creatures you can have.
The second requirement can be met by not having a fixed creature limit per portal, but a limited number of creatures per type. This means that finally being able to attract a dragon to your dungeon means you can get more creatures in total, not just an additional type and this way you don’t have to kick out the rat to make place for the dragon.
However, this adds additional complications. You don’t want the player to simply get all creatures in the pool whatever he does, so you’ll have to add a system to make sure you don’t get a fixed creature composition on a map. A possibility would be to increase the complexity of the attraction algorithms; an example would be to set attraction formula for orcs and sorcerers to be X creatures per training/library room size + Y creatures for the ratio of training/library (reversed for both creatures). Meaning that having larger rooms will lead to a higher number of total creatures but the ratio of the rooms determines the ratio of those creatures and you can’t have the maximum number of both types at the same time.
What would also help with randomizing the creatures you have is by reducing the rate of new creatures appearing based on how many creatures you already have, and increase the rate of new creatures appearing based on your dungeon size and other metrics. This way you won’t have too much front loading and you’ll still get creatures later in the game.
3) Low tier creatures should be useful to have
Of course if you give the player a choice between having 3 strong and 5 weak creatures or just having 3 strong creatures he will go for the former, so technically with the last two requirements implemented you’ve already convinced the players to have his weak creatures stick around. But if they now pale in comparison to the stronger ones the player will still lose interest in them.
The solution is to have synergy between the weak and strong creatures, to have the weak combination add to the fun and effectiveness of the stronger and less common one. This way the player his happy to have them around, and not just for the menial labor they provide.
- * If cultists have summoned a powerful flame demon into your dungeon, have them cast their fireballs on this demon in combat to power him up instead of simply shooting at the enemy.
* A big slow creature can throw small fast creatures at enemies to damage them from afar
* A high damage undead creature with low health can sap the life of other creatures to stay ‘alive’ longer.
I hope this gives some food for thought.