since the X-E3 will come as a last member of the 24mpix sensor product line, it deserves some heavy optimizations:
3.69M-dot viewfinder with 0.85x magnification with a bigger peeking hole and a bigger eyecap so that the sun isnt such a terrible bother as it is now the case of x-e2(s) without a custom workaround
larger capacity batery http://www.solidenergysystems.com
option to lock white balance together with the AE-Lock (Lock_WB On)
render actual focal length of zoom lenses on screen in real-time
usb battery charging (maybe also use of USB3.x for transfers, but thatd require connector with more pins on camera's side)
ability to save a descriptive text memo to each picture
aperture calibration option for manual focus lens (until the camera is turned off) and thus automatic F number detection between opened and closed aperture including halfstops and respective transfer of the F number to exif data (unless there is a way to recognize and register each lens attached using a characteristic light print of each lens on sensor or some other way to recognize any particular lens)
option to select a precise distance for which an AF lens should pre-focus itself (direct setting via phone or tablet remotely and on camera possibly by using arrows as in time lapse digit setting or utilizing the focus ring to quickly dial distance figures in combination with some milimeter, meter and kilometer switch like the command dial wheel on the back, which is usually used to adjust shutter speed or aperture)
focus bracketting option for taking an automated stack of in-focus pics mostly at F4-5.6 to be stacked later (or in camera if fuji feels like its worth the effort), selectable distance range like 0,2m to 00 (something - to - infinity) or 20m to 90m with a confirmation question should the camera be tasked to take 30 consecutive pictures (like this: 30 images needed, ok to proceed?)
if exposure time drops below desired value while in autoISO mode, display that time value in a different colour as a warning (red, yellow, orange, violet, pink, whatever)
detachable corner viewfinder with an approximately 1m long thin spiral data cable to be inserted between camera and evf or camera and other imaging device preferably with calibrated colours and clearly defined color space (placing evf directly to the eye, camera movement becomes 100% independent = no need for flippable rear display anymore)
"attempt" to minimize blackout in continuous shooting coupled with an attempt to lock focus in continuous mode
rolling shutter distortion negation - latency compenstation using the OIS gyro's acceleration data to dynamicaly shift line of data readout from sensor area (probably needs to decrease the video crop a bit to increase sensor area readout for videoframes, limited to lenses with gyroscopes unless gyros installed in the body - there are some actually for the electronic level indicator)
possibly make the thumb-rest with AF-L and AE-L buttons a bit bigger towards back side, so that the thumb has larger area to lay against and feels really comfortable (it is a bit too shallow as it is and requires holding the camera more tightly utilizing whole palm which is fatiguing over long period of time, if i let go, my thumb would just slip along the upper round smooth arc directly under the exposure comp dial. it doesnt have to be superbig, just let it grow 1 or 2mm to the back and shift it 1mm to the right side and elevate the curvature of the plastic part that copies the shape of the thumb 1-2mm to the upside or better yet, merge the plastic thumb-rest with the upper frame in one whole part and the thumb curvature can be then lifted even higher under the exposure comp dial making even more room for the thumb - as of now the thumb-rest is a separate plastic part, this would also allow to place the AF-L and AE-L buttons 1 or 2mm higher for better accessibility without the need to painfully bend the thumb which is the case of current AE-L, also make these buttons a little bit larger area-wise, i think there is enough room for it and wuld be even more after you inflate the thumb-rest to enlarge them about 10% of their current area)
increased diameter of exposure and exposure comp dials so that they are more comfortable to set (could mean enlarging the body ~2mm to the right, but if you implememnt the joystick, the enlargement of the body would happen anyway). a little cutout in the upper frame directly under the main time exposure dial for better access with fingertips wuld be nice (in case of larger dial diameter, it would be enough to cut in the front only, since the dial is shifted more towards the back-side and yes, it would mean there would not be a straight line on the facial side of the camera anymore, but thats the way of ergonomy, unless you want to make the dial twice as high so it can be comfortably rotated with two fingertips using just a gentle force)
it would be nice to have a lens-centered tripod screw at the bottom-side (im not using the mhg-xe, because im worried the arca swiss extension would get in the way of my fingertips while handling the camera as i do and more over itd increase the overall weight which is undesirable for me)
what else..? maybe the ability to detach the mic st1 using a spiral cable and holding it at a larger distance from the camera, making it somewhat more independent and less prone to pick up the hum from the lens stabilizing mechanism and clicks of the dials that would be adjusted on the camera on the fly
otherwise im quite satisfied with the shape, layout and materials of x-e2s, so i guess those micro adjustments in body size would mostly serve for better handling and would not drastically change the look of the body.
edit: yea i knew i wanted something unreal, a (touch) slider (or trackball, cylinder) on the left side of the camera for setting the ISO as smoothly as possible, while aperture and exposure time are both locked
edit2: a metering mode to completely avoid overexposure
edit3: i guess shifting the 4 arrow selector 2mm to the upside will make it more natural to control with thumb once the camera is glued to one's eye..
edit4: option to show a larger and fatter version of the focus+dof scale on the display spanning from edge to edge of the display possibly with precise distance markings at least under each number for more confident manual focusing during video or even single shots. itd be nice if the display showed distance as a figure, which would be especially useful in the vague tiny space between 10meters and infinity, where you cannot tell if you are focused at 50m or at 1,3km or at the moon..