In general, film photography is having a comeback. Prices for used film cameras skyrocketed in the last years for a few models.
Personally, I find photographing on film really rewarding. Having a physical product in the end (be it a print of the image or only the negatives) makes the process more enjoyable. So if you have some old film cameras lying around, I can only recommend giving them a try. Maybe there are even old films with old memories in these cameras.
The problem with retrofitting a film camera back with a Pi camera is that the Pi camera has a dinky little IMX477 sensor which only covers a small, small fraction of the area that would normally be illuminated on 35mm film, so you would not get very good images at all.
If they came out with a full-frame sensor that plugged into the Pi though, that would be awesome.
That said -- that's for 35mm cameras. Now there are also other film cameras ... I am working on using a Pi camera to scan a large format 4x5 area to bring a Toyo view camera back into the modern age :). It takes a good 15-20 minutes to scan the image and I get gigapixel results. Still a work in progress. Un-doing the effect of CRA optimization on the sensor's microlens array is annoying.
Stephen Johnson was playing around with this in the 1990's.
http://www.betterlight.com/field_photography.html
"I initially captured 180 degrees of view in a 6,000 x 40,055 pixel image, but soon learned that Photoshop was limited to opening files with less than 30,000 pixels in either dimension, so I had to perform surgery on the original TIFF file to reduce the image to just under this limit."
That said, and assuming you use a camera module that you can disassemble far enough to expose the sensor, it shouldn't be too hard. You'd most easily I think design and 3D-print a replacement cover with a light-tight fitting to place the sensor on center at the flange focal distance (ie in the designed film plane), and route whatever cables out to where you could connect them. Maybe also a case for the Pi that has a 1/4"-20 screw to mount on the tripod socket, just so you don't have to cram your face past it to get a good look down the viewfinder.
You'd probably have a hard time getting anything like a wide-angle shot. I don't know offhand what sensor sizes are common in RPi camera modules, but I feel like expecting 1"-class would be expecting too much, so you'd be dealing with a pretty fierce crop factor.
To get down into “normal” lens equivalent would require something like 8mm. A fisheye would probably be ok because the center tends to be relatively less distorted. But if there are 8mm rectilinear lenses for 35mm film cameras you probably can’t find one and maybe not afford one...Its price would buy a lot of film.
I mean there are digital sensors that retrofit film cameras. Products for Hasselblad 500 series and Mamiya RB have been around since the early days of digital. They have always been less than $100k and still are today.
Anyway if you want to use an old film camera, just buy some film and have a go. They are incredible mechanical devices and a pleasure in the hand and produce that film look naturally.