Hi, I'm new to the forums. I would just like to post some of my ideas.
More town variety:
I think that there should be port towns, which have docks (obviously this would require ships and sea travel) etc. and a different layout to normal ones, more focused around trade. Larger towns and cities should have more structured grid layouts, while smaller ones are quite spread out. There could also be castle towns, which are surrounded by a large wall and have a gatehouse (and maybe some back entrances). These towns would be well defended, but the walls would be able to be taken down by siege weapons etc.
Also, some of the larger towns should have underground sewers/dungeons (for castles) which you can explore are where prisoners and slaves are kept. Certain civilizations could even being some sort of arena where they make prisoners and slaves fight against captured animals.
There would also be mining towns, farm villages etc. that all have their own layouts and feels (mining towns would have a large mine underneath where miners work to collect stone and ores).
Also, a more simple idea this time. The ability to collapse a tile. If enemies gain the ability to tunnel into your fortress, and you don't have enough time to build a wall, then you could send your miner over to collapse the tunnel, knocking some earth from above to refill the tile. This will remove the earth tile from above, which may make buildings collapse etc.
I have some more ideas, but I don't have enough time to post them right now.