The Cambridge Grammar of the English Language (Page 410) defines "Fused-head NPs" as follows:
Fused-head NPs (noun phrases) are those where the head is combined with a dependent function that in ordinary NPs is adjacent to the head, usually determiner or internal modifier:
[1] i Where are the sausages? Did you buy [some] yesterday? [determiner-head]
[ii] The first candidate performed well, but [the second] did not. [modifier-head]
The fused-head NP is an important syntactic theory proposed in CGEL that draws a different line between the category of pronoun and determinative than in traditional grammar.
CGEL has been around for almost two decades but I doubt that the syntactic theory has been accepted as standard among grammarians. But ditching CGEL's theory and going back to traditional grammar is not an option.
Is there an alternative modern approach to the fused-head NP?