People love stories.
I've never felt something that strong for a story for a while.
Last ones were:
Attack on titan, Squid game, Interstellar, the Joker, Inception.
Mystery:
Why everyone disappeared? Why are they in the game? Who created that game? Why lasers shot and kill people? Why nature is growing so fast?
Mindgame answers:
Level 1: humanity found out how to solve forever death, making humans immortals. they loosed meaning, that's why they created fake virtual worlds where they can die just to feel alive, and give back meaning to their existence.
Level 2: extra-terrestre play with humans, and they are their source of entertainment.
Level 3: all of this is just one guy story playing in his head as he became crazy due to his 2 friends violent death, and developed psychological issues.
Level 4: the firework they saw before everyone disappeared was in reality a meteorite that crashed on shibuya, making everyone in imminent death situations. they all connected in this in-between death-life world, where winning games meant fighting for life.
Theory:
Last plan, we saw a vicious joker card on the table.
One final game to play at the hospital to definitely get back to life.
The joker can be any card, at any time, and ask the player to play the same deadly game as the 7 of heart. Meaning forcing only 1 survivor, multiple times.