What if scenario…
"Vegeta… let's forget the Potara earrings. Let's perform fusion," Goku suggested to Vegeta who was still reluctant to fuse with Kakarot. The last time they performed fusion was against Janemba.
Fusion Zamasu had gotten up after being damaged by a SSB Kaioken Goku and prepared to attack them. Vegeta was reluctant to fuse with Kakarot, for the last time they performed the Fusion Dance was against Janemba.
"He'll tear us to shreads Vegeta! We haven't got much time to spare!" Goku yelled at Vegeta who was very hesitant to fuse with that bumbling idiot again but knew he had little choice in the matter.
"Fine! I'll do the stupid fusion. But only this once!" Vegeta finally agreed with Kakarot, much to his happy expression.
"Fusion….HA!" After a short dance, the two finally fused into Super Saiyan Blue Gogeta who simply stared at Zamasu. As for Future Trunks… he was admonished at the sight of his father and Goku fusing together into a being who may be powerful enough to defeat Zamasu.
"Who are you?" Zamasu asked the new powerful being standing before him. "I am neither Goku nor Vegeta, I am Gogeta. Its over Zamasu, I have come for you!" He repeated the same words he said to Janemba from back then.
Powering up, Zamasu began to attack the powerful warrior. "Every force you create has an echo. Your own bad energy will be your undoing!" Gogeta said, moving towards Zamasu and delivering several powerful blows to him. With such unimaginable speed, Future Trunks and the others including Zamasu couldn't even see his movements.
Two more kicks were then delivered to Zamasu as he returned to the ground and powered up a single ki attack in his left hand. Enraged by this, Zamasu charged at Gogeta who simply tossed the attack at him and punched him square in the face but with little effect. Just after that, a bright light emanated from within Zamasu as he was torn asunder by the awesome power of the Stardust Breaker.
"The fusion between father and Goku... what awesome power!" Trunks noted, Gogeta descending to the ground before the Kais and Trunks. The threat of Zamasu had finally come to an end, thanks to the help of Fusion.
"Trunks, is the time machine still working?" Gogeta asked Trunks who nodded at his question and flew towards where he placed the Time Machine at while Gogeta reverted to his base state while still fused.
Using the Time Machine, Gogeta, Trunks, Bulma and Future Mai safely made it back to the past and just as they returned, Gogeta defused back to Goku and Vegeta.
"Goku! Your back! Is it finally over?" Chi-Chi rushed towards Goku who was worn out after the battle against Zamasu. Vegeta was met by younger Trunks who too wanted to know what transpired in the battle.
"Yeah... it's over. I gotta hand it to you Vegeta, I didn't think you'd agree to fuse with me again. Just like old times huh?" But eh... those words didn't do much other than annoy Vegeta who gave him a menacing glare.
"Ah! Don't bring that up again! I'd rather die than fuse with you again!" Following his remark, Whis agreed to take Future Trunks and Mai back to the future, before the reign of Zamasu took place and ask Lord Beerus to destroy him...