Good Question! The first consideration I would suggest for a camera is material / opacity. I started with black ABS, which is colored with pigment and naturally opaque. Everything about the material is good for a camera application, but the finicky nature of its "printability" and toxicity moved me to PLA. Finding opaque PLA wasn't a problem (or even a thought), until suddenly the supply of black PLA I had in the Schlaboratory was found to be translucent, even though it was called "Black as Midnight". Apparently, in the cut-throat realm of the 3D printer filament industry, coloring the product with a dye rather than a pigment (formed from colored solids) was deemed financially prudent (even though I suspect the margins on the product are princely). And the vast majority of users of the product have no requirement for absolute opacity. I obtained some other polymers to try, but many are crazy-expensive, erosive to non-hardened nozzles, or simply not opaque. My standard is ZERO light through a 1mm sheet of printed filament, with the brightest light I can find behind it. Currently, I am using eSun Black PLA, and it works flawlessly.
Beyond materials, avoiding light leaks that will destroy your exposures is Job One. Essentially a box with a tiny hole, your task as designer is to invent a way to open and close that box, and advance (and possibly rewind) the film, such that the joints and tiny gaps don't let ANY light into the box. That, alone, is one reason that black is a traditional color for cameras. A typical camera light trap requires stray light to change directions three or four times before it can shine into the camera. Any bouncing light gets absorbed, rather than reflected. For parts that fit/slide together, I find that a 0.5 mm tolerance works well with most FDM printers.
TWO KNOBS. So many reasons.
Another thing I would suggest is that 120 film is always going to produce MUCH nicer photographs than 135 (35mm). Also, dealing with the indexing numbers on 120 film is better than most of the indexing options for 135. But, if you want a real design challenge, a 35 mm pinhole camera that works as well as a 120 camera is a lofty goal. I think I designed one, but the fiddly nature of assembly negates the other virtues of the design, IMO.
Also (I'm guilty of this), documentation of some sort is key. Some people will look at the STLs and the pictures of your camera and intuit how it all goes together, others... not so much. I try to make good faith documentation and then be very responsive to questions, for unanticipated questions / misunderstandings.
That's just a quick brain dump. I hope that helps. Don't hesitate to hit me up with any questions you may have in the future.