The notion of a macro-parameter suggests that several grammatical properties should cluster together around one core property, a conception that facilitates language acquisition (see Hyams, 1986, 1992; Liceras, 1989; Hyams and Wexler, 1993, among others) and makes fairly strict predictions about sequence of acquisition, possible and impossible grammars, etc.
In the case of the NSP, there have been several proposals regarding the core property (see Gilligan, 1987, 78–96 for a summary). Taraldsen (1980), for example, proposes that the subject position is potentially bound by AGR, which gives rise to the possibility of null non-thematic subjects. That availability of binding by AGR underlies that-trace filter violations: Since the subject position is bound in NSLs, extraction is licensed.
Rizzi (1982) modifies this notion, arguing that rich AGR binds the postverbal subject position, so the availability of that-trace effect violations in NSLs is related to the possibility of extracting from the postverbal position. Long wh-extraction and null resumptive pronouns also follow from this basic asymmetry: in languages like Italian, the postverbal subject position is licensed (technically, governed) in a way that the preverbal position is not in English. The availability of postverbal positions, in turn, is overtly manifested in free inversion (cf. Sections 2.1.3 and 2.1.5 above). Gilligan (1987) tests the typological predictions of Rizzi's early account, and reaches certain implicational correlations between having null subjects, free inversion and that-trace violations, which Roberts and Holmberg (2010) further refine.