After having generated a number of worlds (but not played in any of them), I've seen a number of new/modified symbols on the maps.
This is what I've seen and what I think they mean:
- Dark grey omega: Mountain Halls
- Light grey 'I': Tower. Cleared of necros. Evil spreading is reversed.
- Oval inscribed with '+':
- Light grey: Monastery
- White: Fortress. Will probably be renamed to Castle, if I understand Toady's comment correctly.
- Black against background: Fort
- Odd P with t or R with decorated leg: Forest Retreat (have only seen a few of them, but one magenta [see below]). How do they differ from other Forest Retreats?
- Non goblin settlements (including destroyed ones [mu character]) changing color to magenta (and back): Magenta color probably indicating being under necro control.
- Light magenta destroyed settlements [mu character]: Probably under necro control again, although I don't see why there's a light and normal magenta variant.
- Dark grey upper case Pi: Dark Goblin Fortress: Not actually new. Stripped of their overlord. If the overlord was spreading evil the spreading is reversed.
- (Light) Magenta Monastery/Fort/Fortress. Probably under necro control.
- Goblin Fortress (but with upper case omega character) spreading evil. I've seen one case where it was destroyed and evil was rolled back. Most likely fortresses where the inhabitants (dwarves or invaders) dug too deep.
- Tombs spreading evil (and have the evil shrink away). Probably a disturbed mummy that then can get eliminated.
I'd appreciate to hear about any further insights and observations.
Edit: I've updated the post based on the two replies below in an attempt to summarize it. Thanks for the feedback.