Seems to me the Goblin King should arrive with an appropriate entourage of hench-goblins. We've tried a variation were any creature drawn as part of the Goblin Kings +3 draws are placed with the GK. However this proved to be rather deadly (especially if you draw 2 or more trolls!)
Another variation we have tried is to keep 3 goblins aside and when the GK is drawn those goblins are placed with the GK. (Banding)
We also said the GK cannot be targetted until all his accompanying creatures are defeated. (Bodyguards)
This game seems so ripe for expansion with extra mechanics and abilities for creatures!