This is a complicated idea and I am working it out while I write this, so bear with me. It's meant to go hand-in-hand with the idea of retiring a fortress so that adventurers can go there: what if the evilness level of the fortress was tracked by the game so that the AI taking over the fortress gauged its hostility to adventurers accordingly? Additionally, truly evil fortresses could alter the alignment of the surrounding area over time even without being retired - perhaps every year the game would check the alignment of the fort, and might spawn a few appropriately-aligned tiles if the fort is becoming very good or very evil...
Good Actions:
- Taming and breeding good-aligned creatures, like unicorns
- killing hostile evil free-roaming creatures (but not caging them and then killing them, that is cruel)
- Freeing caged evil creatures, not killing them but also not letting them leave the map (eg. keeping them in a contained jail cell)
- Using cage traps rather than fatal traps
- Giving merchants unnecessarily good deals
- Letting dwarves have breaks
Evil actions:
-A nonhostile creature dies in a trap on your base, eg. a spike trap
-Taming and breeding evil-aligned creatures
-Freeing caged evil creatures and letting them escape the map
-Killing good-aligned wild animals
-If excessive numbers of nonhostiles die in water or magma on your map, the game gets suspicious and calls you evil
-seizing merchant goods
-letting merchants go insane
-freeing something from a cage, only to kill it
-many more, I'm sure, but the game has to be able to track the event somehow, so "leading merchants into a goblin ambush" doesn't qualify.
Since the player would doubtless be killing numerous goblins, which are evil, the tendency would be for Good Align to rise over time. However, most players seem to tend towards evil actions as well, and there should be enough of those to let a fortress balance out on the evil side eventually.
It seems complex but I bet there could be a lot of far reaching effects to this scheme. What would happen to your fortress if it was truly evil, or truly good and pure? Would you get special megabeasts, or trade with different groups?