This idea has been discussed before but I feel it can be discussed further. Of the Fookchiefed threads below, only the first is truly relevant. The others deal with a similar issue but not the same.
Previous threads:
http://www.bay12forums.com/smf/index.php?topic=47607.msg960652#msg960652http://www.bay12forums.com/smf/index.php?topic=3010.0http://www.bay12forums.com/smf/index.php?topic=6218.0http://www.bay12forums.com/smf/index.php?topic=5056.0http://www.bay12forums.com/smf/index.php?topic=4782.0My suggestion is that upon arrival of a siege when the “vile force of darkness” message appears you should have the option of dispatching a diplomat.
If you don’t think you can quell this siege you can dispatch a diplomat to negotiate. You must have a diplomat in the nobles screen, and he could use all those skills we never seem to use (liar, judger of intent etc).
Once dispatched the goblins will sit on the edge of the map as friendly rather than enemy forces. The diplomat will go to the leader of the enemy force and you will have a discussion similar to trading discussions.depending on the size of the goblin force they will set demands. The can demand either goods up to a certain value total, and or dwarfs upto a total number. You can try and negotiate this down. Once the numbers are settled the goblins go into your fort and take what ever they like (including artefacts if they please) up to the value (as liars they may actually go over the value a bit). They will also drag off any dwarfs they please (may need to make restrictions for nobles and military) upto the value. They then leave.
This would give the gobins slaves, artefacts and loot and create interesting storylines.
If the goblin siege is exceptionally large, and your fortress weak, they could demand you become a vassal. In which case you switch civs, all your dwarfs get extreme bad thoughts (some may leave) and you now get goblin migrants. When army arc comes in there will obviously be major differences.
Exploits: it may occur to whily players that they could send out a diplomat to stall time to get their dwarfs safe, and prepare their traps. Once negotiation is over if: negotiation fails, the goblins turn back enemy kill the diplomat and continue as normal, a goblin is killed anywhere in the process of taking items from your fort all goblins turn back enemy and the siege continues, the goblins fail to reach their totals for loot and slaves, cos the player has locked them away, within a certain time limit they return to siege mode.
Historically sieged cities often paid the invaders to leave.
You may feel this is undward or goblin like to deal rather than slaughter :
http://www.bay12games.com/dwarves/story/tt_zecalo.html
This system could work if elves or humans sieged only without the slaves part. Elfs and humans have different motives and therefore may take different items. E.g. elves cart off all your animals and wood (they then perform funeral rights on the wood).
It could work with mega beasts, but in a different way. I don’t think you should be able to negotiate with a dragon, but you could pull the old maiden tied to a post surrounded by treasure manouver.
You could try to mega beasts to accept tribute by chaining a dwarf to a post outside your fortress with some trinkets of a certain value. The mega beast may accept the trinkets and eat the sacrifice, or may just attack your fort. If it does accept you will have to provide the same sacrifice each year hence forth.