How about a monster vending machine! You pay coin, it spawns a monster or group of monsters (1 selection at a time to avoid overpopulation). Might be the first big coin sink in the game since the AH!
Just to flesh out the idea even more, we have 2 alternatives. First is Promoter Grogan from Tharashk Arena & Crucible. We've ventured to visit him in remote locales, now he comes to Stormreach and brings selections of champions with him to test the adventurers of Stormreach. Second is a nameless summoner, perhaps just The Summoner, a high level mage who is running low on funding for his research. Pay him coin and he will summon all manner of beasties for you to test your mettle. "Red Dragon Kobold, I choose you!" /throws pokeball.