One of my growing interests is in split
syntax.
Though much of linguistic theory has tried to reduce the "rules" of
a language to a small uniform set, it more accurately appears that different
parts of a grammar may operate according to different generalizations.
Split ergativity
has
long been acknowledged. But ergativity-nominativity is just one
phenomenon that can be "split" across different parts of grammar.
For a few more examples:
In Panare, VSO
order appears to be the norm in one tense-aspect; but OVS
is arguably the norm in a distinct tense-aspect (Payne 1994a).
Also in Panare, NP syntax is highly non-configurational,
while main-clause structure could be argued to be really quite configurational
(Payne 1993c).
In Maasai, some of the nominal vocabulary operates
on a lexicalized
semantic gender system; while other nominal vocabulary is
sensitive to a more "on line" cognitive-pragmatic
gender system (Payne to appear d).
What does this mean for theory? In part, I believe it means that
explanatorily adequate theories of syntax must be historically-informed,
acknowledging that change moves gradually through the grammar of a language.