It doesn't seem to be that far fetched. Villains wishing to end their existence and take the world with them are pretty standard adventuring fair, not to mention everyone on this forum has been swooning over every mention that BBS is going to be more serious and mature than previous KH games.
The Unbirths are said to be the predecessors of the Heartless. The Heartless pursue the darkness in hearts and consume them and by doing so they destroy worlds, so I hope it's not too much of a stretch to guess what the Unbirths seek is equally destructive. If they seek to end their existence and the pursuit of this goal threatens the worlds, that's a pretty decent set up for a serious story to be told.
The Unbirths are said to be the predecessors of the Heartless. The Heartless pursue the darkness in hearts and consume them and by doing so they destroy worlds, so I hope it's not too much of a stretch to guess what the Unbirths seek is equally destructive. If they seek to end their existence and the pursuit of this goal threatens the worlds, that's a pretty decent set up for a serious story to be told.