DBZ: Father and son - Future Goku and Future Gohan

Those two were the heroes of Dragon Ball Z, Goku (right) and Gohan (left). Although, they are from an alternative timeline where two years after defeating Frieza, Goku had died due to a heart virus and the world was left without a hero. Six months later, two Androids #17 and #18 had killed the Z Fighters (Piccolo with Nail and Kami, Vegeta, Tien Shinhan, Yamcha, Krillin and Chiaotzu) and they can never be wished back to life with the Dragon Balls because Piccolo and Kami died, thus beginning an era of darkness. This is the timeline Future Trunks is born.

Future Goku
Backstory:

Future Goku was the strongest fighter among the Z Fighters. His life was identical to his mainstream counterpart until Mecha-Frieza and King Cold came to Earth.

On Age 762, after defeating Future Frieza on Namek, Future Goku barely manages to escape using one of the Ginyu pods on Namek, and flies directly to Yardrat, where he spent a more year training to get used to the Super Saiyan form and learned the Instant Transmission technique. A year later on Age 764, Future Goku used Instant Transmission to teleport from his spaceship to Planet Earth and fought both Future Frieza and Future King Cold. Grown stronger due to his training, he effortlessly defeated the tyrants and thus saves the world. After two years of peace on Age 766, Future Goku was infected by a Heart Virus and died from it. He could never be brought back to life with the Dragon Balls due to have died from a natural cause. Sometime later, in Dragon Ball Super manga, Future Goku made his brief appearance in the afterlife in a special chapter, along with Future Piccolo, where he just found out he died from a heart virus. He was 29 years old at the time of his death.

Descriptions:

Future Goku, in terms of power and strength, is far weaker than his mainstream counterpart, due to not training as hard as his present counterpart who was warned about the androids and the Heart Virus. Despite that, he was still the strongest among the Z Fighters, and was constantly stated to would have been able to defeat the Androids, especially by Future Bulma. It was later seen that Present Goku was stronger than Future Trunks, meaning Future Goku was just as strong as well when he killed Future Frieza and Future King Cold, and apparently even stronger two years later.

Future Gohan
Backstory:

Future Gohan was the last of the Z Fighters. His life was identical to his mainstream counterpart until his father Future Goku came back to Earth and killed Future Frieza and Future King Cold.

Six months after the death of his father, on Age 767, after the death of Piccolo, Krillin and the other Z Fighters by the androids, Future Gohan was fueled with anger and managed to turn into a Super Saiyan. He spent 13 years of training, taking care of Future Bulma and her son, Future Trunks, and fighting the Androids whilst saving many people as he possibly can.

On Age 780, Future Gohan fought the Androids in an amusement park, along with Future Trunks but were defeated, which resulted the loss of Gohan's left arm and a scratch on the left side of his face. Future Gohan, grown stronger from the verge of death and intense pain via Zenkai, had faced the Androids as a Super Saiyan, this time without Future Trunks, apparently able to hold his own. However the Androids have combined their strength and overpowered Future Gohan and killed him. Future Trunks found his best friend and mentor dead on the ground, and transformed into a Super Saiyan for the first time (in the manga, he already was able to transform into a Super Saiyan but was weaker than Gohan in base form). He was 23 years old at the time of his death on Age 780, the same age as Present Gohan in Dragon Ball Super during the "Future" Trunks Saga, who became a scholar in contrasts to the fierce warrior Future Gohan turned out to be.

Descriptions:

Future Gohan is far different from his mainstream counterpart. Having grown in an apocalyptic era for 13 years, he is far aggressive and serious, but also kind, brave and has a sense of humor, just like his father. He also wears his father's clothes as a memento from him, even Future Bulma remarked how he resembled his father. Future Gohan is also more of a capable fighter like his father Goku and his mentor Piccolo than then Gohan of the main timeline.

In terms of strength and power, in The History of Trunks manga, Future Gohan in base form was too strong for Super Saiyan Future Trunks. However, Future Trunks, at the time he became a time traveler for the first time, was about at the same level as Super Saiyan Future Gohan before he died, meaning Future Trunks grew even stronger through Zenkai from the latest time he found the Androids and lost before going into the past to meet Goku. And the one armed Future Gohan as a Super Saiyan was stated to be just as strong as the angry Super Saiyan Goku on Namek. With Future Trunks at a level to that of Super Saiyan Future Gohan who got strong as Super Saiyan Goku on Namek, was barely even a match against Super Saiyan Goku who blocked Super Saiyan Future Trunks' attacks with a single finger.

P.S.: Whilst it is theorized that Goku Black, the main villain of the Future Trunks Saga from Dragon Ball Super, may have been Zamasu in disguise (later revealed to not have been the case), or a fake but a darker version of Goku wished from the Super Dragon Balls (unlikely), or Son Goten (ARE YOU SERIOUS???), it is further theorized that Goku Black may have been an entity or an apprentice of the usurper Supreme Kai of Universe 10 Future Zamasu, using Future Goku's dead body, supporting that Goku Black is thinner, less muscular and remarkably younger in appearance than Goku. However, how Goku Black and Future Zamasu have knowledge about Goku due to being in an alternative timeline in which he died of a heart virus.

Son Goku, Son Gohan, Dragon Ball Z and Dragon Ball Super © belong to Akira Toriyama.