Though Toei Animation has yet to officially comment on the news, the team behind Dragon Ball announced that the next Dragon Ball Super film is already in the works as they are doing their best to craft a project that could both top Dragon Ball Super: Broly and explore new territory. It's going to be a tough as it's following up the most successful film in the franchise to date, but it's not impossible.
One of the major restraints on Dragon Ball Super's potential to do just that, however, is that the series will still somehow end up following the end of Dragon Ball Z despite how much Goku and the Dragon Ball world have grown since the franchise has returned with new stories. So the fix is a simple one. It's time to retcon the end of Dragon Ball Z, and this new film is the perfect opportunity to do so.
The famous "End of Z" joins the Dragon Ball characters ten years or so following the end of the Majin Buu saga. In that time, the characters have gotten older and both Goku and Vegeta still train, but seemingly not as much during these times of peace. After fighting the reincarnation of Majin Buu, Goku leaves his family to train this interesting new fighter in order to finally get another strong opponent. This was the official end to Akira Toriyama's Dragon Ball all those years ago, and it still is for all intents and purposes.
Dragon Ball Super picks up a few months after the end of the Majin Buu saga, and the series is meant to fill in the gaps between that saga and the End of Z. That was a reasonable premise when the series first began, but that's not really the case anymore. As fans have seen, Dragon Ball Super has opened up the franchise to gods and beings even stronger than those gods, multiple universes, even more multiple timelines, and has brought both Goku and Vegeta to power levels fans would have never thought possible.
But to think Super still has to lead to the original ending puts a massive constraint on just how much more the series can explore. The ten year gap could still lead to this finale, sure, but it would negate much of the work Super is doing for the characters. Fans talk about ballooning power levels in this series, but what would bother them even more is a gigantic nerf. Just look at the response to Gohan! If he were to do all that work to regain his strength, and then go back to his original state for the End of Z fans would riot.
He's not the only one, either. As characters like Krillin, Android 18, Android 17, and Vegeta (just to name a few) would be underserved by the old ending. But with a new film, the series can finally free itself of the End of Z. It's no secret that series creator Akira Toriyama originally brought the series to an end under a great amount of stress, but now that he's got an entire team helping to bring the stories to life perhaps he'll have a different ending in mind that incorporates all of these new elements.
Retconning the End of Dragon Ball Z may seem like a major undertaking, but Dragon Ball Super's existence already retcons the original series by telling us there were more battles during that ten year period. It's essentially opened the floodgates by existing, and the response to the series over the last year or so has shown that fans are completely willing to accept these new stories.
As for how it can retcon the series? Honestly, there's no wrong answer. Any ending that acknowledges the events of Dragon Ball Super and its future would be a right one. In fact, the series could still take the spirit of End of Z and just acknowledge that Goku's at a God level. Because why would Goku train Buu's reincarnated form when he's got prospective opponents like Jiren, Broly, and literal gods to fight instead?
There's just no point in keeping to a years old ending just because it's "canon." The series has evolved so much since then. There's a solid chance that fans would accept a new ending years from now as long as it meant we got more great Dragon Ball stories.