1st: On rare occasion when you kill a goblin that has his portal open it will remain open and lead to one of several random dungeons or sometimes just a simple room with a resplendent chest and its elite guardian.
2nd: On extremely rare occasions (like uber D from d2 rare) a goblins portal will lead to another one of Azmodans lieutenants. This would be Greed's domain. Where the walls literally bleed gold. And on the very last level of this massive dungeon would be Greed himself looking every bit like his goblin lackeys except for the fact that his is nearly 800 pounds heavier than them and very sensitive about the mound of loot he is sitting on. Maybe he would even like to add your gear to his possessions....
3rd: Add some kind of legendary that only Greed has the chance to drop. Like Greed's Goldskin. Every hit has a chance to make enemies bleed gold for 5 seconds; just like hitting a goblin.
It seems like this would be cool because some of the lore books you get in the game talk about a goblins portal leading to Greed's domain. Please expand on my idea if you've got something.