(First off, I realize that this doesn't fit too well with the spirit of the game or the spirit of dwarves, and as such it would probably be better as a mod, but I figure I should put it here so that it endures criticism to the point where it would fit the game the best even as a mod. If such a mod already exists for this I am thoroughly sorry for wasting your time.)
Ever wanted to annoy the tree-huggers in a way other than simply chopping down trees or flaunting off
useless fashionable wooden mugs and jewelry? Maybe even go to them and burn their forests before their eyes?
I propose that the ability to actually lay siege on other civilizations be added. Not only would it open the possibility of a near entire military (viking style) it would allow the player to make more complex interactions with other civilizations.
The attack would start after you designate a few squads to move to another fortress. You continue to manage your fortress as they prepare to leave, taking plenty of food, weapons and armor, and siege engine components. This would probably be much easier when wagons are added, but pack animals could do. They then depart, and you receive a message days/weeks/months later when they have reached their destination. Because dwarves do not use mounts and are fairly slow, it should take a long time for them to arrive.
After that, its up to you. Crush them with your catapults, dismember them with axes, or fill them full of bolts.
A few additions that might work well with this-
-Ability to send food to your military
-Ability to add auxiliary members such as cooks, siege operators, and medics to your squads, but not as part of the ten member cap
-Ability to station your own diplomats within other civilizations
And again, read the small addition at the top of the post