'Dragon Ball Super' has reached a point where Goku is practically the Deus Ex Machina of the anime. Here are there big reasons why he should not be the one to end Black in the ongoing arc.
With the way the plot of the Future Trunks arc is currently going, it would not be surprising at all if Gohan or Vegeta, not Goku, would be the fighters that would ultimately be the key to winning the fight against Black Goku.
The preview of the upcoming 'Dragon Ball Super' episode seems to allude to a rather melancholic and bittersweet reunion between Future Trunks and his master from the future, Gohan.