In an effort to think outside the box with regard to morphosyntactic alignment, I've come up with something that is kinda-sorta tripartite and kinda-sorta fluid-S, and I was wondering if it is attested in any natlangs.
Basically: the most fundamental opposition marked on nouns in this system is the degree of control that the subject has over the verb.
Nouns have three cases, which currently I'm labelling "ergative", "absolutive" and "oblique".
Verbs can be either stative (indicating, shockingly, a state- "adjectives" also fall in this category) or dynamic (indicating an action or a change of state). This is an aspectual distinction to a degree, but it is determined lexically rather than being a gammatical alternation per se.
Stative verbs are uniformly intransitive and, logically, the subject does not have control over the state. As such, the argument of the verb is marked with the absolutive case:
tana wetɬä
- tan-
- man-
- a
- ABS
- wetɬä
- sleeps
"The man sleeps."
For dynamic intransitive verbs, the subject is marked with the absolutive case if he lacks control over the action or the ergative if the subject has control. For example:
tanu rokʰä
- tan-
- man-
- u
- ERG
- rokʰä
- shouts
"The man shouts (on purpose, in order to warn his friend that a swarm of bees is heading in his direction for example)."
tana rokʰä
- tan-
- man-
- a
- ABS
- rokʰä
- shouts
"The man shouts (involuntarily, perhaps because he has been stung by a bee)."
Moving on to transitive verbs, the same marking obtains dependent on whether the subject is marked as [±control]. However, the patient of the verb is marked with the oblique case:
tana maɬi bätsʰä
- tan-
- man-
- a
- ABS
- maɬ-
- dog-
- i
- OBL
- bätsʰä
- kicks
"The man kicks the dog (accidentally)"
tanu maɬi bätsʰä
- tan-
- man-
- u
- ERG
- maɬ-
- dog-
- i
- OBL
- bätsʰä
- kicks
"The man kicks the dog (on purpose)"
For what it's worth, the oblique also marks the possessor in genitive constructions:
tani maɬu tani ŋʷaka
- tan-
- man-
- i
- OBL
- maɬ-
- dog-
- u
- ERG
- tan-
- man-
- i
- OBL
- ŋʷaka
- bites
"The man's dog bites the man."
Looking at it semantically, it appears that the agent maps to the ergative case, the experiencer to the absolutive and the patient/theme to the oblique. Is there a similar attested natlang? Off the top of my head, I can't think of one immediately, but my knowledge of languages beyond Europe and western Asia is pretty minimal.