Now that NPC civilian professions are being coded in, I think it'd be a cool Bloat-ish thing if a goblin Civ could declare a campaign against your fortress.
A campaign would be something akin to what we do when we start/reclaim on top of a goblin fortress, but with the goblins doing it to us. An army would arrive, but not attack right away, and it would bring civilians with it. They choose some spot on the map, and build workshops, etc., preparing their attack. Goblin woodcutters start chopping down trees, goblin siege engineers would make catapults and ballistae to defend, masons could build walls or tear down your walls, and miners would dig moats or (gasp!) alternate attack routes into your fortress.
If you don't wipe out the goblins, they would start sending skirmishers to try and take out your outdoor dwarves, try to capture "territory" on the small scale by building walls/fortifications/siege engines ever closer to your fort, and eventually start sending in raids to your fortress proper. They could possibly even get reinforcements over time, akin to dwarven migrants.
Obviously, the AI for this stuff would be pretty simplistic (start by going for some area a large distance from fortress entry points, random sorts of timers for attacks, and they wouldn't worry about food, bedrooms etc.), but it could still be really fun. It would need to be an extremely rare occurence, since it would probably last several years of game time, so they probably wouldn't happen more than once every 10-15 years.
What do people think? Does it sound like a good idea?