“The Kyon Theory” is actually pretty popular in the fandom and Nagaru slips a single comment that might point to it in one of the latter volumes, albeit through Koizumi, which predictably makes it extremely ambiguous as he says it’s possible and at the same time says he doesn’t think it is the case.
I haven’t read Gatsby, but I understand what you mean. What I was going for wasn’t about Kyon’s protagonism, which is pretty much indisputable, but Haruhi’s irrelevance as a character once Melancholy ends. Koizumi, Mikuru, Yuki, and hell, even Tsuruya are sporadically (and in Koizumi’s case, almost surreptitiously) given development or at least a reasonable amount of limelight without the need of a specific series of events being centred around them. Koizumi is the best example as nothing so far has had him as the central focus, yet he still manages to be developed a bit.
Haruhi, on the other hand, is just… there. If she’s lucky. Sometimes she’s not even there, and sometimes she’s not even responsible for what happens. It’s hard to even consider her a secondary character but more of a “wild element” as you said. I just have trouble referring to her as an “interesting character” like Cless mentioned because, to put it bluntly, she’s more of an object than anything else. Just reading any of the conversations Kyon has with Yuki or Koizumi about the strange shit that goes around makes it clear how she’s usually referred to as a phenomenon rather than a person.