The Aggramar/Sargeras video reminded me of something when Sargeras said "The circle nears completion. The mortals must not disrupt the rebirth".
When Arthas approached the Frozen Throne Ner'zhul said "Return the blade and complete the circle". I wonder if these are connected, or just a coincidence.
Most likely it just means Sargeras is getting his body back. But could this be a "Keep them busy so Ner'zhul can take control of Bol'var" thing?"
Sargeras wants to wipe out all life. The scourge does a good job of that, Ner'zhuls rebirth could be his final gambit.