@Riamus
Your post made me think of this. It's an extension of your idea.
Use heat as a spawn modifier. using 100% as the top level for spawning screamer.
People post regarding more biome spawn.
There are two spawn frequency governors.....maxcount="1" respawndelay="2.9".
As the heat rises, the maxcount goes up and the respawn delay goes down.
These can be calculated or transposed to real time, well close to it anyway.
Heat gets checked anyway, so no additions there, but with each check as it raises
to 100%. There could be a plugin value that increases spawn rate and volume in the
area, if it hits 100% then screamer spawn. Screamer death starts cool down. After
dispatching the main screamer spawn it turns off checks on devices already being used,
but checks on any new devices turned on or heat generating activities after cool down
has begun.
Simulating attracting zombies because of activity, in a constant and gradually elevated
manner, more proportionate to the activity in the area, ending with a screamer spawn.
So it would use heat to elevate walker activity, still be dependent on gamestage, the
thing that may and may not be liked is later game would render stronger variants.
Then just have an on/off selector for heat gen spawn or not. Off crafter's fantasy, or
crafter's nightmare. Also since there are so many different heat generators in the game,
it could possibly give some life to spawn. The more activity in an area the more the
external spawn proportionately increases, and when you leave goes back to normal.
I did something similar before as an event, walking on terrain did nothing, walking on
blocktypes other than terrain built heat, and raised spawn. So in cities walking/riding
on the street, increased activity. You wanted it to cool down, you hid or walked in the grass.