Indeed a replacement for the Image Viewer could improve it in many aspects.

But for me the most important features I miss from the original Image Viewer are not related with rotation support but:
- Fix the annoying issue that is not remember when you only want to show pictures from the camera and not other images.
- Add support (maybe optional) for zoom/unzoom with swirl gesture.
- Add option to disable zoom/unzoom on rocker buttons (so they can be used for volume control while using the image viewer).
- Add OSD control to change from fullscren to windowed screen instead of just tapping (very annoying sometimes).