Thoughts on Goku Black's Extreme Power

Geekdom101 is one of the most-well known personas among the Dragon Ball fan community that provides insight on the franchise. He recently uploaded a video explaining the reason for Goku Black's enormous power in Dragon Ball Super:

To give a little more clarity as to Zamasu possessing Goku as Geekdom101 tells it:

- Beerus used the Super Dragon Balls after the Champa Arc to wish Universe 6's Earth to be restored. So when Zamasu learned of the Super Dragon Balls, he cannot gather and use them at that time.

- Zamasu kills Gowasu (without Beerus interfering) and obtains both the Potara Earring, and the Time Ring.

- Using the Time Ring, Zamasu moved forwards into the future so the Super Dragon Balls become active again, and thus gathered and wished to switch bodies with Goku.

As the Super Dragon Balls have a cooldown time of 1 year before reactivation, we can surmise that 2 or so years have passed after the Champa Arc. So there is plenty of time for Goku to train and become stronger. Add that to Zamasu's fighting prowess and he has developed a new Super Saiyan form unique to Black, while also surpassing even Super Saiyan Blue. 

It is also noteworthy that in the earlier episodes Chichi mentioned that she used up all the money given by Mr. Satan, forcing Goku to return to farming work. Either the economy on Earth is that bad for Chichi to use so much money, or she has a case of Oniomania (aka Compulsive Spending Disorder).

Future Zamasu was the one who wished for immortality for the simple reason that in the Future Trunks timeline, Goku is already dead. But he still has the same ideals as the Prime Zamasu and thus continues with the Zero Mortals Plan.

And as the most recent episode has shown, Goku Black was from the Prime Timeline, but was protected from the Time Paradox caused by Beerus for killing Zamasu, thus allowing him to continue to exist. This is due mainly to him possesing the Time Ring.