What’s the meaning of 会 in 你明天这个时候会是在做什么?
你明天这个时候会是在做什么? is not a very fluent sentence construction. I would have re-written it as 你明天这个时候会做什么? instead.

会 is not useless. It carries the meaning of "likely" in this context.


  What will you be (likely) doing at this time tomorrow?


The way sentences are expressed in Chinese is not the same as it is done in English. What appears to be redundant in English need not be redundant in Chinese.

In the above example, because you can't predict what another person will be doing tomorrow, 会 has to be included in the Chinese sentence.

In the English sentence, the phrase "will... be" already expresses the meaning of likelihood in the question.