Dragon Ball Z is one of the most well-known anime titles of all time, and all it took was adding one letter. However, the series had a working title that was actually completely different, and may have changed the course of Dragon Ball history had it stuck.
Dragon Ball Z is the name of the anime adaptation of the second half of the Dragon Ball manga, where there was no separation into two parts. While no clear reason has ever been given for the name change, it's hardly uncommon in anime--just look at Naruto Shippuden. Since there is a timeskip between the end of Dragon Ball and the beginning of Z, perhaps it was felt that they needed to emphasize this was a new chapter of the story. It seems that part of the reason why there was some feeling that the title needed to be updated was a misunderstanding that Gohan would become the new protagonist.
Dragon Ball Z's Original Title Put the Focus on Gohan
The Anime Team's Working Title Was All About Gohan
According to an interview from Shenlong Times issue #5, translated by Kanzenshuu, Kenji Shimizu, a producer at Fuji TV, indicated that the working title for Dragon Ball Z was actually Gohan's Great Adventure. It's not clear from the context if that was a subtitle, and Dragon Ball was still meant to be included, or if that alone was the title. Apparently, the producers of the anime worked with the title Gohan's Great Adventure for quite some time before speaking with Toriyama, who went on to emphasize that Goku was, indeed, still the main protagonist. Toriyama then casually suggested adding a "Z" to the end, and Dragon Ball Z was born.
They, like many, thought that Goku growing up and having a son meant that the protagonist role would be passed down, similar to Boruto and Naruto. Goku's early death in the Z era no doubt helped solidify that opinion, even knowing that he could be brought back with the Dragon Balls. It was only Toriyama's clarification that kept them from changing the title, as it seems they were ready to move forward with it, by all indications.
Would Gohan's Great Adventure Have Succeeded Like Z Did?
Dragon Ball's New Title May Have Changed a Lot
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One can't help but wonder how the new series might have been received if it had kept the Gohan's Great Adventure title. The apparent shift in focus from Goku to Gohan may have caused a stir in the early fandom, but what's more interesting to think about is how this title might've affected Toriyama's work. If Toriyama knew that the anime title emphasized Gohan, would he have featured Gohan more than Z does? Might Gohan have actually stepped up and become the new lead character, like he was at one point meant to?
It's impossible to say how much influence a title change would've had on the show's popularity, but a snappy title like Dragon Ball Z certainly helped on some level. As long as the story content didn't change too much, it seems likely that Gohan's Great Adventure would still have been a hit, thanks to Toriyama's engaging writing.

Dragon Ball
Created by Akira Toriyama
First Film Dragon Ball: Curse of the Blood Rubies
Latest TV Show Super Dragon Ball Heroes
Summary
From the creative mind of Akira Toriyama, Dragon Ball is a mega multimedia franchise that spans back to the 1980s. Dragon Ball expanded quickly, starting as a serialized manga for Weekly Shonen Jump in Japan. It made its way overseas via manga and an anime adaptation that is enjoyed worldwide. Dragon Ball was the initial starting animated series that followed the adventures of the young Son Goku as he sought after the Dragon Balls. These mystical orbs would grant the wish of any who gathered them together. Then, the series would branch off into the immensely popular Dragon Ball Z, which followed Goku as an adult and featured high-intensity battles and Goku's never-ending search to be the strongest. The series has also enjoyed several popular video game adaptations and continues to release several new animated series and theatrical films up to the recent popular Dragon Ball Super: Super Hero.