Unknown World of Anime

In the world of anime, there are different stories, plots, and awesome characters. But ofcourse, the story wouldn't be complete without freakin' amazaing (well, sometimes maybe not XD) MAIN CHARACTER.

But what if one day.. Different protagonists from different worlds of anime found themselves in one UNKNOWN world together?

Who gets to be the main character then? Will one of them gets a chance to take the lead role in a completely unknown world?

But most importantly..

HOW ARE THEY GOING TO GET BACK TO THEIR OWN WORLDS?