From reading reviews of the advanced screening of Battle of Gods in Japan it sounds very similar to one of the most popular fan fics ever made and thats not just including DBZ but just about anything. Dragon Ball AF inspired multiple other spin off fan works and by many fans of the original series was more loyal to Akira Toriyamas work than Dragon ball GT ever was.
For those not familiar with Toybles Dragon Ball AF the story takes off years after the DBZ Manga.
Dragon Ball AF summary
- Goku beats Freeza – this triggers events which create a powerful enemy
- Goku is in another dimension and to get him back the team need 7 Super Saiyans to perform a ritual
How Battle of Gods pans out
- Goku defeats Freeza – triggering the awakening of Birusu
- For Goku to get strong enough to beat him they need 6 Super Saiyans to charge his powers
Although these are only two points so far they are very similar events which makes me believe that Dragon Ball AF may have inspired the Battle of Gods story.