I'm interested in the historical linguistics of constructions like "that's me" versus "this is she" when answering the phone. Searching online led to a Google Books peephole view of a book that describes this as happening with the transition to late modern English (Denison, English Historical Syntax). Unfortunately since I don't have other access to that particular book, I can't tell where exactly Denison's periodization places the transition to late modern English (1700? 1800?), or whether he has a theory about what process took place.
Browsing online suggests two possible theories to me. (1) This is just part of the centuries-long process through which English has been losing its case structure. (2) The emphatic pronoun, which happens to have the same form as the nonemphatic objective pronoun, became preferred. (This post on english.SE suggests this theory, but he admits he doesn't have evidence.) I suppose both 1 and 2 could be true.
To fill in the picture, it would also be interesting to know which form authors such as Jane Austen used, and whether or not there is evidence that the archaic form was consciously revived by prescriptivists after it had died, or simply survived in certain set phrases.
If one of these theories is right, how does it deal with the following? I ask "Where's the basketball team?"
*That's they over there.
?That is they over there.
To my ear, the first one sounds like nothing a native speaker would ever say, while the second one might pass. Does the lack of a contraction make it sound more emphatic (theory #2), or less formal (#1)?