Andrew McInnerney was recently published in the journal Language, with his paper "Specifying coordination in extra be sentences". Andrew argues that sentences like 'That's the thing, is we lost' have a structure parallel to that of similar sentences containing colons, as in 'That's the thing: we lost.'  He offers evidence that both types of sentences involve ellipsis, and that the 'extra be' optionally arises as part of the ellipsis process. 

Specifying coordination in extra be sentences

Abstract: I argue that the phenomenon of ‘extra be’ (e.g. That’s the thing, is we lost) can be analyzed in terms of specifying coordination. Specifically, ‘extra be’ derives from a ‘colon phrase’ (:P) structure, where Spec-:P is a host sentence and Comp-:P is a pseudocleft adding information to the host. ‘Extra be’ arises when the head T of the pseudocleft raises to :, an operation that is possible only under specific circumstances involving ellipsis in the pseudocleft. I motivate this analysis by first considering a set of syntactic, prosodic, and semantic properties exhibited by extra be sentences, including properties of ‘extra be’ itself, properties of the post-copular specificational phrase, and locality conditions in the construction. I then develop the analysis described above, emphasizing in particular the assignment of a uniform structure to both extra be sentences and their ‘extra be’-less counterparts (compare: That’s the thing {: / is} we lost). Finally, I compare key features of the new analysis with those of previous proposals.