My Theory.

[ruby2579]

TOTALLY agree I have a feeling if they showing the necklace in that specific coffin means that could it possibly be his mother in that coffin?

[Princess of Darkness]

I think it's possible that it's his mother-- or, I've also thought, maybe it's the original doppelganger-- the original petrova. Another idea that has come to mind-- is maybe, the necklace is key or is the key to opening the coffin?

My husband speculated that eventually, the necklace will be spelled, and will have to be placed on Klaus-- which in turn, would somehow "freeze" him, almost the way a daggered original is frozen. Then, he'd be placed in the coffin, and whatever magic that is sealing it now, would seal it again, this time for good. Then, ironically, Klaus would be trapped in a coffin for all of eternity, just as he'd done to pretty much all of his family.