It sure seems to me as if we can discover new sui generis properties, and that this has some important consequences for the approach to ontology, which I am very sympathetic to, outlined in Carnap’s “Empiricism, Semantics, and Ontology.” Some examples of discoveries of new sui generis properties: that feeling I had when I was writing the verses of Sophie thinking about Alyssa; Dennett's example (from “Quining Qualia”) of discovering that the sound of the open E on a guitar is, in part, composed of the sound of the E harmonic at the 12th fret might also work (especially if "sounds like open E" was a sui generis property before, and so the structure of my linguistic framework is more severely changed around by the introduction of "sounds like the E harmonic at the 12th fret"); also, the first time one tries any really distinctive kind of food, esp. a raw ingredient. Really, any time we use demonstratives like “this experience” because other words just wouldn’t do the trick. (One thing: how do I know the properties are sui generis right off the bat? Well, I don't, especially since I take calling them "sui generis" to amount to a claim about the place of the predicate for the property in the L-structure [i.e., undefined and probably also undefinable]. But I discover them and add them at least to the model at once [even if I only pick out the properties demonstratively], and I can try to reassure myself that they are sui generis later.)
What exactly do I mean by "discover"? I discover a property when I acquire a concept of or else actually refer to a property that was not previously in my ontology, in the model of the language I was using. This property is introduced all at once, not after subtle reflection and meta-reflection on previous experience and theory. It is precisely because it is all at once that I do not know that the property is sui generis, though I think I have my suspicions from the get-go.
What exactly are the consequences? Well, that the changes to an L-structure that we can effect are sometimes completely determined by what seem to be universally adopted methodological norms. It seems like, just in the face of these novel experiences, I have to add the new property to the model of my linguistic framework. There is no room for conceptual engineering at this point (though that can come later - perhaps "sounds like E-12th fret harmonic" later becomes a defined term). Also, a sort of realism about properties - not that we need to quantify over predicate symbols, but that (at least some sui generis) properties are in some sense "out there". They are not necessarily "out there" in that our norms should be such that the property should have been in the model all along; this is not a claim about the inadequacy of our norms, but about the sensitivity of linguistic change to extra-linguistic forces.
At least that’s what I want to say the consequences are, but maybe I’m wrong. In ESO, Carnap says only that in a “linguistic framework” the use of existential quantifiers must be rule-governed. But my discovery of these new properties of my experience (if it was as I describe it) was quite rule-governed, in the sense that I did not violate or change any rules of English semantics in pronouncing it. Perhaps adding a property to a model does not change the rules of a language. It seems to depend on what counts as a “linguistic framework.” If numerically identical frameworks can have different universes of discourse (perhaps relative to a single context) or words (say, predicates coined ad hoc to pick out newly discovered property), then I do not necessarily adopt a new linguistic framework whenever I denote new sui generis properties. I have been thinking of linguistic frameworks as interpreted L-structures, but perhaps this is too rigid. Perhaps we might conceive of linguistic frameworks in such a way that they might be constituted, in whole or in part, by rules for introducing new terms, or even new “kinds” of terms.
I guess I still don’t really have an answer to the question, at first a bit inchoate, that motivated this post: Does the apparent fact that, in our linguistic framework – scientific English? –, we can denote (what we sometimes correctly take to be) entirely new sui generis properties somehow entail that ontology is not guided by the convenient choice of a linguistic framework?
In other words: sometimes we introduce altogether new (sometimes sui generis) properties into our ontology by a process that apparently does not necessitate commitment to any sort of heavy background theory. The introduction, and the need for the introduction, presses on us with a feeling of utter immediacy. Clearly, we are following some norm here – both in feeling the need for the introduction, and in actually executing it (an act perhaps beyond our control). This norm, whatever it is, is meta-ontological. It dictates how and when to introduce new kinds of terms and objects. The question is: can this norm be stated within or somehow compassed by the meta-ontology of ESO, say as a peculiar fact about the rules that constitute the linguistic framework of present-day scientific English? It seems hard to square the utter non-verbalness of the need to follow the norm and ESO’s (perhaps deceptive) appearance of making all ontological norms essentially semantical rules, and meta-ontological norms reasons for changing semantical rules. It all seems to depend on the nature of linguistic rules and linguistic frameworks.