Dragon Ball Z expanded the mythos about the alien race known as Saiyans - but Dragon Ball Super took things to another level, by having Goku and Vegeta tap into the power Super Saiyan Godhood. Of course, Goku and his friends first had to unlock the power of Super Saiyan God in order to protect Earth from one apocalyptic threat: Lord Beerus, the God of Destruction for Universe 7.
Since DBS's first "Battle of Gods" arc, Beerus and his angel attendant Whis have become friends with Goku and Vegeta - but what if that friendship isn't meant to last? What if Lord Beerus is actually Dragon Ball Super's one true final villain?
It needs to be remember right from the outset, that Beerus once promised to destroy the Earth. After his fight with Super Saiyan God Goku, Beerus destroyed a small rock and fell asleep, having expended a lot of power himself. He's since become friendly with Goku and Co. and Earth in general - pretty much because the Z-Fighters keep bribing Beerus with elaborate feasts, whenever he visits Earth. That truce has held out for some time, but Beerus could come back to his initial promise at any time, for any reason.
The other big thing that's been hanging over Beerus' relationship with Goku and Vegeta is the secret history the God of Destruction shares with Freeza. Beerus had Freeza destroy Planet Vegeta on his orders, because the God of Destruction hated King Vegeta. To Beerus it's barely a blip on his the radar of his memory - but if Goku (and more importantly Vegeta) find out, the friendship between them and Beerus could quickly be over.
Return of F
With the secret connection between Freeza and Beerus still in play, having Beerus be Dragon Ball Super's final big bad would almost certainly involve having to resolve Freeza's storyline, as well. If Dragon Ball Super is looking for a worthy event to bring Freeza back for, a fight between the Evil Emperor, the Saiyan warriors, and the God of Destruction, would pretty much pull together all the elements that got fans invested in the series in the first place.
The Mortal Threat
The other reason that Dragon Ball Super fans often theorize that Beerus will be the series' ultimate big bad, is that the path of Goku and Vegeta's story in the series seemingly makes that destination inevitable. Both Saiyan warriors have advanced into power-up transformations that officially qualify as divine; for Vegeta, that's Super Saiayn God Super Saiyan Evolved, and for Goku, its the power of Ultra Instinct.
Ultra Instinct has been a particularly big powder keg, as the gods of the multiverse were legitimately intimidated by Goku achieving the mastered form of the technique, during the Tournament of Power. With a mortal now achieving the power of the gods, Goku is all but provoking a response from those gods. After all, Beerus' original vision was a prophecy of fighting a godly Saiyan - maybe he was just early in challenging Goku during "Battle of Gods".
The other factor of Dragon Ball Super's mythos that makes Beerus and Goku having a final battle seemingly inevitable, is the nature and purpose of a Destroyer God. Destroyers are the gardeners that prune threats to their respective universes, in order to protect and preserve life. Well, at this point it's arguably just a matter perspective on whether or not Goku and/or Vegeta are threats to Universe 7.
The Saiyan Warriors' battles have indeed threatened entire worlds or the universe (and now the multiverse) on numerous occasion - and as stated, they're only getting more and more powerful to the point of soon outpacing the gods. It wouldn't be surprising if those gods deemed the existence of Dragon Ball Super's Goku too great a threat to the multiverse to allow him to remain on the mortal plane.
Full Circle DBS
As you can see, the dots have been connecting in a lot of ways, to bring Dragon Ball Super full-circle back to where it first began: with Beerus having to face the prophecy of battling a god-like Saiyan, and possibly destroy that warrior and its world. Of course, having seen the full Dragon Ball Super saga now, Beerus and Goku's battle will be much more dramatic and emotional for fans than what we got the first time around. And based on fan discussion throughout the series, there's one outcome that fans would want from the second round of Goku vs. Beerus:
The Next Destroyer1comments
Dragon Ball Super fans have been theorizing since the series' firs arc that there can only be one culmination to what "Battle of Gods" started: a new Destroyer God being anointed for Universe 7. Fans each have their pick as to who should step up to that position (you know who yours is), but there's one thing for sure: whomever it is, Beerus wouldn't pass the mantle without a good fight.
In fact: in the end, testing out a potential new Destroyer God through combat may be the entire point of the fight. We'll take a Beerus-Goku-Vegeta-Freeza tag-team battle for the title! Especially if Vegeta is the new Destroyer!