So, with the backdrop in place- the question is:
How much of a varying degree must a Zelda game have for you as a Zelda fan to not consider it a copy, clone or opportunistic game that feeds off previous titles?
I don't think it is, or will ever be, a matter of the degree but a matter of the execution. That might end up being a fairly subjective way of determining it, but if so then so be it.
Mostly I just feel that Twilight Princess is rather awkwardly designed at a lot of points. It's difficult to describe. The Oracle games were solid constructions without as many perceptible flaws (well, Ages was; I dislike Seasons more than any other Zelda game), and all the copying they did off Link's Awakening was very intentional, and very well-executed. Phantom Hourglass is one of my favorite Zelda games and had very original gameplay, and I saw Spirit Tracks as an attempt to reverse what Nintendo perceived to be PH's flaws. If Ocarina of Time had been a highly flawed game, I doubt most people would have any gripe about it being replicated (for example I would be totally in favor of a remake of it, if that were the case, and I normally detest remakes). When a concept has flawed execution, you don't mind improvements on the idea. Usually derivatives are considered bad when they can't stand up on their own in terms of production value. Ironically I think Spirit Tracks made things worse with its attempted improvements, but that's irrelevant here.
I actually do think Ocarina of Time had some flaws, and I feel that Majora's Mask refined them. Whereas, Twilight Princess has weird pacing and awkward art design (as I see it), and it makes mistakes in its following of Ocarina of Time. If it had matched Ocarina of Time's level of quality or surpassed it, I wouldn't have much or any qualms about it being derivative in terms of how it was made.
I think another thing here, though, is fanservice. Aside from the gameplay and game design, you also have matters of characters, setting, and storyline. I feel like Twilight Princess pooled ideas from Ocarina of Time's world in a shallow way, rather than a natural way. Again this is ripe for disagreement, but that's how I see the game in the end, and that, combined with its production values, are why I fault it for being similar to Ocarina of Time while I wouldn't fault another game for it.
(If it seems like I sorted my thoughts out and got more confident in what I was saying somewhere around the middle of this post... that's because that's exactly what happened.
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