Seed of Worlds surprised me in his Shiny TTRPG Links 74 by pointing at an old post from here, That Goblin Tribe where a goblin group survives and strives by avoiding close combat and only offering ranged and prepared fights.
What if we forgot this scenario and went in another direction? Some seeds.
If fairyland is parallel to our lands but time flows there at different speed (different direction?) Could goblin infestation be the result of fairyland goblin overgrowth spilling into our realms?
One symptom of "those goblins are not from here" is inadapted weapons and tactics. Straight out of fairyland, the goblins have no experience fighting humans. Their warfare is goblin-only or faery-only.
There are two kinds of goblin troops emerging from the fogs of fairyland, losers and scouts. Losers have been displaced by other goblin tribes and have had no choice but to leave the fairyland for the hazards of our non-fairyland. They come in in complete tribes and try to settle. Scouts are swift warriors and they are on a reconnaissance mission, be it for losers or for winners thinking that non-fairyland might be a promised land ripe for expansion.
The "losers" case would fit the classical FRP setting where goblins are a convenient cannon fodder, but with something behind that vanilla surface.
The "scouts" case is fun for skirmishing and can be reused, and developped into a full invasion of goblins.
Swimming up-river to the source of infestation there in the fairyland would prove an interesting, character-warping, adventure. What is pushing the goblins that push our losing goblin tribe?
An interesting scaling down of the fairyland spilling over pattern would be the fairy-land peninsula variant. An isolated piece of fairyland is inhabited by a goblin tribe. It is linked by two or three "wormholes" to our non-fairyland and the goblins exploit those trapdoors to raid for necessary resources.