Distortion live preview and initial placement, scale and rotation marker.
A lot of reference points in maps without rigid or rotated CRS or of insurance map type only become easily apparent after transformation in map overlay. Would be great to have a live preview of the referenced map (preferably realized in 2D-Canvas) to continue referencing with the information already entered.
There might also be a complex initial marker type with interactive handles for position scale and rotation to get started with maps of unknown or known non-north orientation.
draggable handles would be O,N and Scale where O is the position(x,y), N the orientation, or rotational component around origin/z-axis, and Scale (scale at point) as some kind of increment/decrement interaction with scale updated in 1:###.### format.
Scale at point should also be able to entered manually when dpi is known and scale retrievable from image or metadata.
Max Friedrich Hartmann commented
Once 2 or more control points are known, scale and rotation components of first complex marker could be overwritten by derived calculated values, then 1st marker becomes regular control point.