Enter the Nikoverse

The game would have involved 5 major players. Niko, Biko, Grey Niko, Jiko and Chancebot. Biko, Grey Niko, Jiko and Chancebot get sent to Niko's dimension due to a crack in all five dimensions, which acts as a portal of some sort.

The game is about Niko trying to get the other four home. Each area in the game is dedicated to getting a specific character home. This is the order that they are sent home.

Jiko

Chancebot

Biko

There is one major twist in the plotline. As Biko is about to be sent home, Grey Niko stops them, and insists that He is sent to Biko's dimension. Niko declines. After this, Grey Niko absorbs the sun, giving him immense power, and he would have been the final boss, which would have been immensely hard. After being beaten, Grey Niko does not die. He instead becomes a part of the sun as it returns to the sky, which would have been a setup for a possible sequel if it got popular enough, which I don't have nearly enough to say about but it's still worth mentioning.

Enter the Nikoverse Epilogue

The main plan would be that the sun is the main antagonist, as it would be controlled by Grey Niko, and it would have attempted to destroy Biko's dimension (this is why he wanted to go there in the original enter the nikoverse). After defeating the sun a supernova would have occured in Biko's dimension, killing him and everyone in his dimension. That is the end of the story.