Ok, expanding on the space monster idea a little, i wrote out behavior in pseudocode-- i don't know if all these actions are possible. What i'm trying to achieve is a state were unexplored chunks of the galaxy grow gradually more dangerous over time. Weak monsters will not uncommonly leak out from the "wilds", and powerful monsters will rarely come out. The more powerful monsters move around less, to give you a decent chance of mounting defenses.
Code:
Abandoned Drone factory
* won't leave system
* on turn 1 produces between 1 and 5 abandoned drone fighters
* produces a drone every 10 turns
Abandoned Drone fighter
* 2% chance of leaving system per turn
* relatively weak
Code:
Death Nebula
* 25% chance per turn of moving
* Will destroy solar trees, and krill
* Attacks populations without “environmental encapsulation”
* Destroyed when enters a system with black holes, or neutron stars
Code:
Solar Tree
* Appear at blue or white stars
* won't leave system
* 4% chance of producing a floater
* 2% chance of producing another solar tree
Floaters
* If system contains gas giant 30% chance of producing larval space squid.
* 35% chance per turn of moving
Larval Space Squid
* 75% chance of moving
* If system contains krill, “eat” krill and advance to Space Squid
Space Squid
* If system contains krill, “eat” krill and advance to Huge Space Squid
* If system has a blue sun, 10% of producing a solar tree
* If system has a white sun, 5% of producing a solar tree
* If system doesn't contain hostile 60% chance of moving
Huge Space Squid
* If system has a blue sun, 20% of producing a solar tree
* If system has a white sun, 10% of producing a solar tree
* If system doesn't contain hostile 40% chance of moving
* Some unusual circumstance upgrades to Leviathan
Space Leviathan (renamed dragon to mesh better with other names)
* Extremely strong
* If system doesn't contain hostile 4% chance of moving
Code:
Small Krill Swarm
* If system contains asteroids
* 5% chance of upgrading to Large Krill Swarm
* 20% chance of moving
* If system doesn't contain asteroids, 60% chance of moving
Large Krill Swarm
* If system contais another Large Krill Swarm, delete both and produce Plauge of Krill
* If system contains asteroids
* 10% chance of spawning a Small Krill Swarm
* 10% chance of moving
* If system doesn't contain asteroids, 50% chance of moving
Plauge of Krill
* Attacks planetary populations
* If system contains asteroids
* 10% chance of spawning a Small Krill Swarm
* 20% chance of moving
* If system doesn't contain asteroids, 40% chance of moving