Implement automatic micro fine tuning for every lens. The reason we pay big bucks for the best camera bodies and lenses is to be able to resolve the finest details in our images. Especially for telephotos I'm finding I have to micro fine tune adjust (MFA) every lens. Even then, when you buy a new body or lens it is a bit of a crap shoot whether everything is going to work or not. The cost of the camera companies improving their quality to the point where the error tolerances are so low that these problems disappear would be prohibitive. To their credit they have given us MFA functionality - now make it easy to do across zoom focal lengths and different focus distances.
Please make my camera user programmable so I can customize it to work exactly the way I want. A good example is taking focus stacks, which currently requires a quite cumbersome process of commanding the camera from a phone to pick a focus point and increment the focus between shots. While you're at it, publish an SDK and encourage third parties to publish and sell scripts (also known as apps) to run on the camera. Enough of this silly reverse engineering such as the folks at magic lantern are doing.
Add a locking mechanism to the lens mount so I can't accidently release the lens by pushing the big release button while I'm carrying the camera. That big release button sure is handy when I'm wearing gloves but it is risky when carrying large telephotos on a rapid strap.
It sure would be nice if the camera could automatically focus on eyeballs when shooting PDAF. Any eyeballs, not just human ones.
I would love to have a swiss arca L bracket integrated into the body.
Add a shutter just inside the lens mount that closes whenever you remove a lens and opens again when a lens is replaced. This would make it way easier to change lenses in hostile environments.