It's a question with typical subject-verb inversion. Statement: That would be a good idea. Question: Would that be a good idea?
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It's a question with typical subject-verb inversion.
Statement: That would be a good idea.
Question: Would that be a good idea?
'that' is the subject. It refers to something outside the bounds of this sentence. In other words it refers to an idea which has already been presented in the given context.
'would' is the verb, actually an auxiliary