That sounds like something I might enjoy. I have considered my time well-spent on brief portions of certain philosopher's writings that I had stumbled on, like Nietzsche, Dostoevsky, Kierkegaard, Camus, and Piaget. I can't even remember who exactly had which ideas, but the amalgamation of many of them have formed a large part of my most cherished notions, so I cannot claim all of my philosophies as purely my own in this regard, even if they are my own takes and modifications of their ideas. I have had much inspiration from the "greats" throughout history, even in my extremely fleeting experiences with them. I wouldn't mind digging a little deeper, but I can be a little stingy with my time, and only like to spend so much of it at once.
You're correct. "Non-existent" isn't quite the right word for it, but there are many non-physical things that I could say exist, like air, sound, and smells, and others that do not, like plans, intentions, and fears. I'm having trouble coming up with the right word for these "non-existent" things, but they are creations of the human imagination, and thus, have no substance, no universal awareness or experience of them.
The idea of the cinnamon bun does not exist, just as the imagined sight, imagined smell, and imagined texture of it does not exist. Ideas do not exist. You cannot share an idea, you can only share the explanation of an idea, since it was created by your imagination, and only exists in your individual mind.
Misunderstandings in communication often occur because people forget that ideas are imaginary, and not universal. One person's idea of what a cinnamon bun "should" be cannot be pointed at, smelled, or heard. It can only be explained through vocalizations and drawn imagery, creating a similar, but not the same, imagined idea of what the first person's idea of what a cinnamon bun "should be" in the second person's mind. The idea couldn't be shared, only roughly translated, because it was imaginary.
I suppose that might suffice for my idea of "non-existent": Imaginary. Whether it exists outside of your imagination, or only within.