Just wish someone would bring back a really basic 35mm SLR like a Ricoh KR-5, but without the meter. Don’t reinvent the wheel either, just copy the camera, part for part. No meter, then no problem with the battery. Include in the box a cardboard exposure calculator.
The one extra expense, premium item in the camera would be a glass penta prism and really good bright focusing screen and diopter adjustable eyepiece. For through the lens focusing you just GOT to have the best viewing available.
When I sold cameras in the 70,s and customers had problems with unsharp pictures 50% was blur from camera movement and the other 50% was because they couldn’t see the screen well enough to focus accurately.