Before Raditz blasted to Earth to see how his little brother was fairing in the first episode of Dragon Ball Z, Goku and the other Z Fighters seem to live in relative peace, though with Goku and Piccolo having a long-standing rivalry that had reached somewhat of a stalemate for the last five years. The arrival of the Saiyans and the revelation of Goku's ancestry was the inciting incident for the rest of the series going forward, with many future story elements tied to either Goku or the Saiyans.
The big question that lingers over the series, even decades after its conclusion, is how the story would have played out differently had Raditz never arrived on Earth, alerted Vegeta and Frieza of the existence of the Dragon Balls, or been given sufficient warning for the impending arrival of Cell and the Androids.
10 Gohan's Future Would Have Been Radically Different
Of all the characters in Dragon Ball, none were more radically changed by the arrival of the Saiyans than Gohan, who was nothing but a happy-go-lucky four-year-old child with aspirations of being an orthopedist when he grew up. That all went out the window, however, when he was revealed to have extraordinary powers that outstripped even Goku and Piccolo, who were the strongest fighters on Earth at the time. Without Raditz, this power may have remained dormant for the rest of his life and he may never have become a fighter in the first place.
9 Dende Would Never Have Come To Earth
Without Piccolo and Kame's death, the loss of the Dragon Balls, and the impending doom of Vegeta's inevitable return, Krillin, Bulma, and Gohan would have never had to travel to Namek to revive Kame and regain the use of Earth's Dragon Balls. By extension, the group would never have met Dende, who would replace Kame after he and Piccolo fused back into one being to fight a newly hatched Cell. While the fusion may have still occurred, this would have resulted in the complete loss of Dragon Balls with no replacement guardian of Earth to recreate them.
8 Piccolo Would Still Believe He Was A Demon
When the Saiyans arrived on Earth, Goku wasn't the only character to have a revelation about their true origin, as Piccolo and Kame were also revealed to be aliens from even further away in the universe than the Saiyans. Before this time, Piccolo and Kame were believed to both be separate incarnations of the same nameless demon who wished to be the guardian of Earth hundreds of years prior.
This demon heritage is the sole reason why King Piccolo's minions, the Dark Vassals, look so different from other Namekians, though this was later retconned to them being mutated Namekians instead of demon spawn.
7 Piccolo Wouldn't Have Become A Hero
This comes back to Gohan's involvement in the Z Fighter's battles, but without Raditz releasing Gohan's hidden power and Piccolo kidnapping the young boy to train for a year, Piccolo would never gain the soft spot for humanity that later resulted in him becoming a hero, as well as the most powerful of the Z Fighters after the Saiyans. Gohan's friendship changed Piccolo for the better in many ways, even if he would still revert to his grumpy, somewhat violent tendencies from time to time, such as when Cell first emerged and his training with the Z Fighters on King Kai's planet.
6 Goku Would Likely Never Become A Super Saiyan
Although it is entirely possible that Goku might have become a Super Saiyan in another battle, this seems unlikely due to how impactful the battle of Namek was with Frieza. Aside from trying to save Earth from imminent destruction and preventing Frieza from obtaining immortality, he was also battling the man who killed his entire race and, eventually, his best friend. Considering the sequence of events would still allow for the Androids and Cell to attack Goku in the future, he may have simply transformed later in life, though he would be much weaker at that time.
5 Cell Would Be Much Weaker
Cell's strength, and by extension the Android's strength, was directly tied to Goku's power levels as an adult, as they were designed under the assumption that Goku would be consistently growing stronger over the years. Therefore, Doctor Gero made a point of creating them to be significantly more powerful than Goku. Without the arrival of the Saiyans, Frieza, and the lower strength level of the Z Fighters, Cell would likely be significantly weaker even in his perfect form and wouldn't have the power to survive in any environment gained from Frieza's cells.
4 The World Would Have Been Destroyed By Cell Anyway
Even though Cell would be much weaker in this alternate universe, he would still be more than capable of destroying the Earth, as the only fighters with any real chance of victory would be Goku and Piccolo. As Goku would not only have not trained under intense gravity for several years, he would also have potentially not gained the Super Saiyan transformation or trained with King Kai.
While Piccolo might stand a chance if he fused with Kame, Gohan was the key piece in his battle and, without Gohan as a fighter on their side, it is more likely that Cell would have won the Cell Games and destroyed the Earth.
3 The Absence Of Trunks & Its Consequences
While some characters never become fighters or obtain their absurd power levels, some characters would never even be born. Had Raditz not arrived on Earth, Vegeta and Nappa would likely have never discovered either the Dragon Balls or Namek, which would also mean that Vegeta and Bulma would never have met. If their son Trunks was never born, Future Trunks would not have been around to warn everyone about the androids and Goku's heart virus. As a result, Goku may have died of a heart virus and the incoming villains may have been much harder to defeat.
2 Goten Might Be Much Older
For the most part, Goten was only used as a way to keep children fighters in the series after Gohan grows up, though, with the absence of Raditz, he may have been born much sooner than he otherwise was. Since Goku was training or dead for the majority of Dragon Ball Z before his "final" death at the hands of Cell, there wasn't much time for him to be a father or husband. Without Raditz, he and Chi Chi may have had Goten much sooner in their lives.
1 The Human Z Fighters Wouldn't Be So Useless
While the human Z Fighters were able to keep pace with the aliens for a long time, as they weren't that far behind Goku and Piccolo, this changed shortly after Raditz's arrival. Once the Kaioken and Super Saiyan techniques came into existence, many of the Z Fighters just stopped trying to keep up. It can't be understated just how overpowered these forms are since they can multiply the character's power by hundreds of times, which makes it very hard for anyone who can't use those techniques to keep up with someone who can. Without the Super Saiyan form, many Z Fighters who opted not to train for the Cell Games, namely Krillin and Yamcha, may have actually become much more powerful in this alternate timeline.