It's impractical for Future Trunks to go back to his timeline and use SSG to fight Black, because of the form's time limit.. The other Saiyans could've used to God ritual to turn trunks into a super saiyan God which would give future trunks the ability to draw off of its power even after the ritual had worn off just like goku during the.. You are correct; the required amount is 6 saiyans, which should be easily achievable in the past (Goku, Vegeta, Gohan, Trunks, Goten, Pan, and Future Trunks are all options, 7 options > 6 options). The real reason the SSJG ritual isn't spammed is because of plot.
Was Future Trunks: Super Saiyan God a missed opportunity? Post by DBFanTalk » Mon Oct 05, 2020 10:43 pm Dragon Ball Super revealed, upon Future Trunks' reintroduction, that he had become an apprentice to the Supreme Kai. The manga explored this by the exposition that he underwent a ritual. At best this acted as a plot device for the character.. Trunks obtains Super Saiyan God in Dragon Ball Heroes, so why not in the Goku Black arc? (In fairness, I recognize that this development came after Super's broadcast.) I am not sure what Toei was thinking with SSJ Rage - SSJ Blue x Kaio-ken was dope and fits the context of the lore, but the former was literally never explained.