I won't pretend I'm a specialist on the topic, but my understanding is that these invading waves from central Europe people, including Mongols, were pillaging food from farming settlements as their main mean of subsistance, which was also their strength, because they could devote themselves entirely to war rather than hunting/gathering or farming. Then they 'disappeared' because they took power over farming people and were absorbed by their civilization... just like their culture was weaker than the culture of the people they conquered.
That would be quite a proper behavior for goblins, no?
That's a very fair assessment, especially of the Huns.
The thing is, that DOESN'T describe how goblins behave at all, where this seems to be even more a matter of cultural warfare than it does racial warfare. (In fact, goblins seem to be the most egalitarian of all races - survival of the fittest, regardless of race.)
The Huns and the Mongols assimilated into other cultures because, once they had pillaged the "civilized empires", they rather liked the lifestyle, and abandoned their nomadic ways. Goblins, however, attack because they are trying to prove the superiority of their violent kill-or-be-killed way of life. They babysnatch to bring in fresh blood. They seige to gain new resources and new land and spread their culture. When and if they conquer, they change the way of life of their conquered to match their image, they don't change their way of life to match that of the conquered.
edit: The real problem is that, in order to be a THREAT to other civs, you need population to spare. That means food for your soldiers-to-be. The great conquering empires of history arose from places where there were overabundances of food. Egypt and China and Babylon had the great fertile rivers. Greece rose to prominance when it had grain, and died down when it overtaxed its soil through chopping down so many of its trees for its navies.
Rome had very fertile plains along the coast of Italy, and the entire reason Hannibal Barca could not defeat Rome, in spite of Rome's every attempt to lose to him, was that he simply couldn't out-kill Roman birth rates or destroy enough Roman farms to matter. Rome literally just ignored him, and sent its troops out to sack Carthage while letting him pillage as many farms or villages as he could visit.
The Huns invaded because they had such a large amount of people after a glut of food production that when famine came, they were forced to invade neighboring lands just to eat. The Mongols were fairly similar (and most historians believe that the Huns were basically just cousins of the Mongols, anyway). The Vikings set out because their lowlands started sinking into the sea, and forced them to seek new places to find food or money.
These conquering barbarian hordes, however, all have a common theme: their numbers swelled when they had excess food, and then they invaded when they could no longer sustain their numbers. While it's possible we can have goblin waves come out as a means of "population control" - either they conquer new lands and eat their food, or they die trying, and don't need to eat, anyway, this doesn't really reflect how goblins behave...
Goblins, are a constant threat, the sort which takes the real organized farming to feed constant numbers that you can send out into the fray. This means you need the infrastructure to be able to GENERATE 80 new expedable recruits a year that you can send to get slaughtered meaninglessly at the gates of a dwarven fortress every single year. And those 80 goblins a year need to be fed for 12 years before they're "adults" and can fight. That means there's 960 goblin child mouths to feed in one location, to keep up a constant stream. Oh yeah, and that's just the children to replace the soldiers... you still need hunters and snatchers and possibly some metalsmiths and miners to get that metal.
That's, of course, not even assuming the number 1 cause of goblin deaths are other goblins...
Basically, either this game is going to need to make individual goblins far more powerful, so that they aren't a fecund zerg rush race, or they need to have a serious infrastructure to be able to pool enough food to train these massive militaries that they can constantly field.
(Of course, if we are talking about WORLDGEN strategies, the number one killer of Goblin civilizations is the fact that goblin civilians have a birth cap of 10 children, and the goblins that get civilian jobs tend to keep them and survive, while their children tend to die in wars before they can have children - meaning that all the goblins who survive wars are sterile thanks to an arbitrary birth cap, which tends to lock goblins and elves, who never die of old age, into a situation where they have no children ever again... although goblins can mitigate this problem somewhat with snatching more children who aren't arbitrarily sterile. Elves tend to become arbitaririly sterilized if they have too many wars because of this, and it really does seem to be almost a bug that parents keep killing their entire race by sending out all their children to die while they stay safe on the farm, even though they can't have any more children.)