My opinion: Challenging embarks with lots of enemies can be done by adding more invader civs, or triggering force event several times. I once made a little mistake and my kobolds triggered so many ambushes that over 400 units were on the map. For me this means that what you want to write already exists in a way.
The issues with it would also be FPS death. Thats why I rather use a single megabeast-sized creature in a civ, than have 3 less dangerous civs attack me.
The other issues is goblinite, which gives "free" ojects to the players, and hurts FPS as well. ~10 items per invaders (2gloves, 2 shoes, 1 helm, 1 armor, 1 pants, 1 weapon, 1 shield, 1 shirt, 1 trousers. Maybe ammo. Maybe socks/shoes under the armor). Thats why I rather use naturally armed and armored invaders, which do not drop items.
I think this is much more interesting:
I might make an optional separate script to tag invader items and remove them from the game when they die so that you don't get too much item clutter. Configurable to control the level of clutter acceptable, what items get deleted, maybe delete all invader items and create some generic "goblinite" tokens on top of their bodies instead, etc.
You can use it for civs with magical items that disappear, or natural armor that is supposed to be part of the body (rock armor on an Elementalman, or carapace shield on an Insectman), or simply allow the option "No Goblinite", and delete all invader items.
It would also allow "No robbing of caravans", if you can manage to delete all items inside a wagon when its scuttled. It would be great if caravans that spawn, and are killed, would also be plundered by the invaders. It would make much more sense than having them die and giving you free items after the invasion left. If caravans are killed, their content is destroyed by the collapsing wagon. But I dont know how to handle this, if the player himself wants to rob a caravan.
My suggestion for your siege-plugin: Allow something like spawnunit, create a number of new units, but with some additional options:
- Units can be hidden (Ambush).
- Units trigger the SIEGE status.
- Units can be thieves or babysnatchers.
- Units have equipment. At least armor, shield and weapons.
- Units DO NOT have to be a civ member. You dont need to spawn a CIV:EVIL ambush... you could spawn a CREATURE:Whatever ambush.
- Units spawn at a random location on the map-edge, either surface, or cavern 1,2 or 3.
This way you can make your own custom sieges or ambushes. Hack down trees? Spawn 5 Ents/Treants that avange the forest. Meddle in dark arts? Spawn 1 fully equipped Lich with 10 skeleton minions. Reach a certain wealth? Spawn a group of well trained and well equipped Adventurers/Raiders. You are bored and want a challenge? Challenge the goblin king to a duel, spawns 300 goblins, fully armed, well trained, plus their King/General with legendary combat skills. Modders could create custom challenges, instead of endless waves of the same invaders.
Bonus points for:
- Units can have custom natural skills. (Like SWORD:0-15)
- Units can have mounts.
- Units can be of mixed creature ids. (Like WOLF and BEAR and ELK fighting together)
Obviously I have no idea how feasable this is, but thats my 2 cents.