If you think about what had been done already, vog was an epic battle with beings from another time, ce was an epic battle with beings from another dimension, there's not much room for escalation and doing the same would be too repetitive. Let's also look at the name "The Arena" this sounds to me like some sort of "games" type scenario, much akin to a coliseum type layout, and the "Queen's Brother" boss would fit well with this theme and the fact that the house of wolves heavily involves the Queen. I'd also be more excited about fighting something small, quick, nimble and unpredictable rather than a big hulking boss with a predictable attack pattern...