I finished The Three Super Saiyans, Broly, Bojack Unbound, and Destined Rivals today. I'm gonna go back into the Android Saga tonight to unlock Grandpa Gohan before continuing through the Majin Buu saga
But right... you haven't unlocked the three What-If sagas? You can unlock them by winning the special battles in the three main sagas.
In the Saiyan Saga scenario, beat Raditz with Piccolo in the "Mysterious Alien Warrior" chapter to unlock "Fateful Brothers". That scenario focuses on what would have happened if Raditz suffered amnesia from fighting Piccolo.
In the Frieza Saga scenario, beat Dodoria with Krillin and Kid Gohan in the "A Pursuer Who Calls Death" chapter to unlock "Beautiful Treachery". That scenario focuses on what would have happened if Zarbon hears about the power of the Dragon Balls and betrays Frieza in an attempt to use the Balls to gain eternal beauty.
In the Majin Buu Saga, beat Gohan with Goten in the "Training With Goten" chapter to unlock "Destined Rivals". That scenario focuses on what would have happened if Gohan weren't able to compete in the World Martial Arts Tournament due to injuries suffered from training with Goten and Majin Buu were stopped sooner by Goku and Vegeta.