Last seasons we've been having same pattern repeating in few dramas, or motif, some similar line and elements. As I said, I don't mind same plot-line, because every drama offers a different approach to the same topic. But looks like writers just can't think out of the box when speaking about the general settings.
Last season was plagued by better or less good time-travel motif. I liked Rooftop Prince version, other people didn't, they liked other dramas, I didn't. It happens.
This year, we have humanity's fate at stake.