Noticed there wasn't a thread on this, I reakon this could be great discussion topic for quite few chapters to come.
So I'll start things off.
Regarding whether or not Kabuto can obtain souls sealed in the death god now, could be a plot twist. Kabuto stated he's surpassed the 2nd and Orchirmaru in using Edo tensei so until we understand what Kabuto can do with the jutsu differently compared to the others its hard to cross out possible candidates of whose in the mystery coffin.
I also thought of possible connection with this mystery coffin and the one the third stopped from being summoned during the leaf invasion at the start of the series.
For those who don't remember it's here, it was a long time ago.
The third also shit bricks over that particular mystery coffin, like Madara did in this case.
Do you think is there a connection?
Edit: Well looks like that mystery coffin the third hokage encountered was meant to be the forth hokage, but due to his soul sealed with the death god he cant be accessed for edo tensei.
But I still belive, it may be someone we have never seen before.