What If?
What if the fusion dance did not exist?
Nothing much would happen, actually. In Movie 12, however, Gokou and Vegita wouldn't be able to defeat Janemba. However, it wouldn't affect the actual series because the only people to use the fusion dance were Goten and Trunks, and they didn't do much with it. However, Buu probably wouldn't have absorbed Goten and Trunks individually and wouldn't become stronger, so Gohan would most likely be able to stop him rather quickly.
What if Gokou lost to Vegita during their first battle?
Vegita would have destroyed the Earth, which means everything on it as well. Because of this, the Dragonball Z series would have come to an abrupt end, and Vegita would have went about his planet destroying ways. This would also affect the Furiza saga, because no one would get the password for the Dragon Balls [ Dende gave the password to Kuririn and Gohan who be-friended him. Without them, Dende wouldn't have given it up ].
What if the Namek Dragon Balls didn't require a password?
Another series ender. Furiza would have gotten immortality, and with that Namek-sei would have come to an end. Furiza would rule the universe, and not even a Super Saiya-jin would be able to kill him.
What if the Super Saiya-jin legend wasn't true?
Gokou would have needed to be stronger at that time, obviously. His power was nothing compared to even Furiza's at 50%. Of course, Kuririn could have done a multi-kienzan, and slice up Furiza into multiple pieces, which would have resulted in a victory[ I guess, unless Furiza could fight like that ].