However, Goku doesn't care about looking elegant, whereas Zamasu does. Goku would probably use that horrible Gi of his, and it would look ridiculous and silly on Zamasu.1) It wouldn't look any different, Goku Black is basically just a skinnier Goku with a different outfit.
You need Sayan cells to grow stronger after each fight. Zamasu's body doesn't have Sayan cells, thus Zamasu Black wouldn't grow stronger after each fight.2) Black appears to possess the power of both Goku and Zamasu so Goku would likely retain his own power.
Black was still corrupted by some of Goku's most iconic traits, such as his great desire to be the strongest (which not all Sayans have). Goku would probably start to believe that he is superior to mortals, being a Supreme Kai and one of the greatest that has ever lived.3) The Saiyan desire for battle is biological whereas Zamasu reasoned out his hatred for mortals on his own.
Goku would lose. As i did say before, Goku's body is far stronger than Zamasu's body.4) Goku could kill Zamasu before he has the chance to realize what's going on and adjust to his new body.
Besides, we did see what would happen if Goku were to possess Zamasu's body. He wouldn't stand a chance against Black Goku. The answer to your "What If" scenario can be found in Episode 61.



