The story up until now has been in the second world.
Yuno travels to the third world in order to repeat what she did in the first world.
That means that the Yuno that we've been seeing up until now actually originates from the first world.
It's not possible for two people to win the survival game.
Therefore, Yuno, after winning the first game, travelled to the second world in order to be with Yukiteru again.
She wanted to repeat this process, possibly forever.
-- now comes the good shit
The moment they jumped to the third world, Eleventh was working on the future diary systems.
His diary, the watcher, saw the contents of Yuno's diary, which shows that he loses the survival game and dies.
Because of that, he discards the idea of doing a survival game.
This causes a chain of events that changes everyone's destiny.
When Yuno dies, Yukiteru becomes the god of the second world.
The first world is most likely gone, and the third world is where everyone is happy.