So was thinking about this today, occasionally you'll get a Goblin Babysnatcher or a Kobold Thief running around your fortress mucking about. Those are really cool random events and such and they're very obnoxious with their "not setting off your traps"-ness.
In thinking about these two though, I've come to the conclusion that such a system, if expanded upon, could provide a stable cause-effect way for sieges to happen, ambushes to occur and be the catalyst for various other things.
This is all based on a positive-negative point system, as the civilization gains knowledge of you they'll gain positive/negative points based on a series of things, also some civilizations start with a base negative/positive with your civilization set in the Raws (IE: Dwarves and Goblins/Kobolds start hating eachother). Certain general actions cause negatives or positives with said race (Killing members of the race is a negative, trading with them is a positive)
Your Fortress garners a general rating system to all civilizations, which contains points in various things.
Knowledge - Their general knowledge level of your civilization, this unlocks later tiers of actions.
Racial - This is a base modifier of their feelings towards you that all races start with in relation to others.
Historical - Events in history will modify this over time, meaning you could start a fortress that begins being hated by a nearby elven establishment based on previous records.
Security - A general estimation of your military strength/traps, the higher this is the less likely they are to act, with lower however they'll be acting more often against you.
Wealth - A general estimation of your value, as seen by enemies (aka hide your stuff in vaults), the higher this is the more likely they'll send out Thieves or Spies against you.
Population - Amount of people living in your fortress, determines a lot of things (how many military units they use, how often Raids will contain Slavers/Kidnappers)
Foreign Relations - Sometimes whether one civilization likes you can determine whether another hates you, the whole "Trading with my enemies" or "Killing my allies thing" does hurt your reputations occasionally on a smaller but still important scale.
Ethics - How your ethics relate to theirs.
Religion - How your religion generally compares to theirs.
Success - How happy your people are, generally this modifier is lesser than others but can cause certain civilizations to hate you or love you more, out of jealousy for example.
Examples of Civilization Units/Escalations
Scouts
"A scout has been spotted gathering information!"
These are the earliest tiers of units, besides your home civilization, these will be sent out by literally every civilization in the area, and some farther, the farther the civ the longer it will take before a Scout may reach you. Scouts overtime generate simple knowledge of your fortress (be it good or bad) for the parent civilization, they eventually leave. They are hidden unless standing within 2 squares of a Dwarf or Animal, they do not trigger traps. Killing or Capturing a Scout generally causes a minor annoyance to the associated Civilization, they will most likely deploy a second Scout afterwards.
Spies
"A spy has been spotted snooping around our fortress!"
These are the step up from Scouts, if something you've done particularly peaks the interest of a civilization they will send these out to investigate. They are hidden unless standing one tile from a Dwarf or Animal, and will not trigger traps. They gather more advanced information of your Fortress (Ethics, Wealth, etc.) and are the precursors to all future actions by the Civilization. They will also find and note any traps they come across and future attacks will try their best to avoid them.
Diplomat
"A diplomat from (civilization) has arrived!"
If the civilization builds enough positive points of information about your Fortress then the Civilization will deploy a Diplomat to your Fortress, a Diplomat will make trade agreements, allow you to set when you'd like to see their caravans, and in general just build a solid reputation with you. They will revisit on occasion and let you know things that are annoying them. They are not hidden and will not trigger traps.
You can send out your own Diplomats to other civilizations to attempt to give them tribute or other actions to try to stay their hand. This will most likely be a new Noble type that will be ordered through his settings.
Thief
"Enemy thieves are attempting to steal our goods! Stop them!"
If the civilization builds enough negative points of information about your Fortress and finds your Fortress has any kind of wealth, they will deploy Thieves to steal it. They are the current variation of Thieves (Hidden, don't trigger traps), however they can show up in squads.
Kidnapper
"Our foes seek to steal and enslave our people!"
If the civilization builds enough negative points of information about your Fortress and finds your Fortress is overflowing with citizens, they'll deploy Kidnappers or Slavers to steal them, both function about the same (Hidden, unless carrying a Dwarf), and can trigger traps. They sometimes show up in squads. Slavers tend to be more physically capable compared to Kidnappers.
Assassin
"Enemy assassins lurk in our Fortress!"
If the civilization builds enough negative points of information about your Fortress and finds your Fortress houses many nobles, they will deploy Assassins to murder them and try to sew chaos. They sometimes show up in squads. Assassinations will cause a negative thought in most Dwarves (except enemies of said Dwarf) and if the Assassin can't find a Noble to kill they'll find the nearest Dwarf available. They are hidden, do not trigger traps, and are capable fighters.
Saboteur
"An enemy saboteur is wrecking the place!"
Saboteurs can appear if you have negative points with a civilization, they show up sometimes in squads and will flat out wreck everything they find. They do not trigger traps but will dismantle them if you don't catch them, and will tend to break everything they can find just because they can, mostly targetting important structures in an attempt to weaken your fortress. They are hidden.
Raid
"An enemy raiding party has been spotted!"
If the civilization builds enough negative points of information about your Fortress, and have done successful thieving missions, they may deploy a Raid, a Raid is an attempt to rob your fortress of all its valuable stuff. Raids deploy weak melee units and large numbers of "Raiders" (Thieves that aren't hidden and do trigger traps), and escalate to heavier parties as time goes on. Raids can occur fairly early on if the Fortress is not well protected against Thieves and Spies. Raids will stop when 60% of the attackers have been killed or incapacitated, when so many items have been obtained (proportionate to how many attackers are alive), or if the Raid has gone on for so many days. They will not kill your whole fortress but will destroy random things (Chests/Cabinets/Beds/Bins/Statues).
-An alternate Raid can include large numbers of Slavers with melee units, coming to kidnap a certain number of Dwarves, as opposed to trying to steal items.
Warparty
"Arm our men, the enemy has gathered a warparty!"
This is the basis of you pissing off a Civilization, if you obtain enough negative points with a Civilization they will deploy one of these against you. Think of them as a mini-siege, they usually carry decent-good attacking units, some Raiders/Slavers, and usually a Building Destroyer or such. They will attempt to break into your fortress and kill/enslave whatever Dwarves they can find until either x% of the attackers are dead, your whole fortress is killed/enslaved, or they Warparty has gone on for x days.
Warhost
"An large warhost has arranged itself on the horizon! Gather your weapons!"
A large version of a Warparty, these are equivalent to Sieges nowadays, they include a large number of enemies varying from slave soldiers to monsters, and generally contains many Building Destroyers. They will include some Slavers/Raiders and will not stop until x% of their Warhost is destroyed, they kill/enslave your entire fortress, or the Warhost has gone on for x days.
Siege
"The enemy has besieged us! Fight to the last Dwarf!"
A massive assault sent forth by Civilizations that absolutely and utterly hate you, they usually include dozens of base attacking units, tons of advanced units, many building destroyers, and shit tons of Raiders/Slavers. They're end goal is to make sure your civilization is wiped off the map, sometimes Sieges can be led by the Civilization's current leader. They will not stop until x% of their troops are dead, they kill/enslave your entire fortress, or the Siege has gone on for x days.
Reinforcements
"Allies have come to our aid! Fight on!"
During an attack by an enemy army (be it Warparty, Warhost, or Siege), once such an event has occurred after a delay allied civilizations with high positive points will send out armies to assist you, these armies will arrive late into the fight most likely (if the fight ends they'll just turn around and go home), but depending on your reputation can be a very sizeable force and can be the determining factor of who wins a siege. Sometimes these parties can include the leader of a civilization if your points are high enough.
Caravan
"A caravan from (civilization) has arrived!"
Typical caravans as they are ingame currently.
Liaison
"A liaison from (civilization) has arrived!"
Typically come with caravans, but a Liaison will try to make trade arrangements for the coming years including taking notes on what goods you have in your stockpiles and requesting to know what you want in future caravans to trade for. Sometimes Liaisons come bearing gifts from the civilization.