My parents have a huge collection of slides from my childhood and before, and I have recently "inherited" a huge stash from the previous owner of my house. It's kind of a pain to view slides, especially in a lounge environment, and it's slow and boring to scan them too (typical slide scanners take 1-2 minutes per slide, although at 5000+ DPI). However, HD video projectors are actually pretty cheap, and I already have one. So, what about a device with an LED backlight, a digital camera, and an HDMI output to use the projector I already have? How about if that was something like a raspberry pi, and it saved every image that it projected onto SD too? I should end up with a full-HD slide viewer, and a higher-resolution (around 2000 DPI) image file (depending on the camera).
So far I have a form-factor, roughly the same as a pringles can, but square. Slide backlight at one end, slide slot just in front with a microswitch in the bottom connected to a GPIO pin. Unlike a normal slide viewer I'll just be taking a still image and projecting that, instead of video. That should get me access to the high resolution modes of the camera (>1080P). I did some experiments with my Canon 20D and a macro lens, which were... OK, but not great. My current alternative is a Sony 8MP USB webcam designed for fitting to a stereo microscope. They are about $70 on ebay. I'm still not totally clear on how to make it focus from a few inches away, but that's why it's a project, right? :-)
So the rest of the shape for the viewer is taken up with an optical-bench type arrangement, since I don't know yet how far away the camera will need to be from the slide surface to get a full-frame image. Wherever that is, the Pi will go directly behind it. I'll add a few buttons to the case to allow you to cycle through already-captured images, and possibly a second SD card accessed via SPI, so that the images can be removed without crashing the Pi.
At the end, the aim of the game is to be able to have "one last real slideshow" of all these slides, with a byproduct being a stack of SD cards of 8MP-ish images of them all.