That is the end of the main story currently... Believe it or not, that's where it is...
There are 3 areas you can go to fight enemies, depending on your hunter rank. If you've reached / beaten that boss without having the other options in the sortie menu then you need to fight more enemies to rank up.
There are also a large number of videos / images that have been shared here which are NOT part of the game. They're side stories / alternate scenarios that are just uploaded as videos / images, like the ones posted one page back.