In a recent lore interview, Steve Danuser spoke about Malfurion sacrificing himself to allow Ysera to return from Ardenweald, explaining that it wouldn’t be forever.
Not Without Great Sacrifice
During the Dragonflight alpha, we datamined dialogue that suggests Malfurion may choose to sacrifice himself so that Ysera can return, taking her place in Ardenweald.
Malfurion: I must be the one to stay.
Malfurion: Ysera knows this — though she would never ask it of me.