The premise of my question is wrong, and was based on spending too many hours trying to explain the ins and outs of Chinese verb structure to a colleague. I'll leave the question here anyway, though, because the answers that were provided below may be useful to others. Thanks!
It seems that Chinese employs many strategies to avoid the phrase ”会是“. Indeed, 会是 makes no sense, although following the logic of (very) basic Chinese grammar, it should mean "will be". Other stative verbs can take 会 to be used in the future: 会在，会有，会想. So what is special about 是 that precludes its use with 会 to talk about the future?
Two examples of sentences that use "will be" in English, and my translations of those sentences. These examples seem to reveal a set of strategies that Chinese uses to work around the usage 会是.
- "Next year, this place will not be my home.", I would say: 明年，这个地方就不是我的家了 -- but this seems to add extra meaning: "next year, this place won't be my home anymore.
- "next year, I won't be a student." 明年，我不会当学生。 -- but this avoids the use of the verb 是。
- "next year, I will be a student." 我明年是学生（了）。 -- but this lacks the focus on future-ness that the English sentence conveys, and that could be conveyed in Chinese with ”我明年会当学生“。