As newer cameras more often deal with stabilization and horizon correction by itself (better or worse) it is less urgent case, but for RAW material this would be also a nice feature, it is something like the correcting horizon scenario mentioned in rotation section for Hugin example (watch Youtube Tutorial for Hugin).

Horizon correction
It could be handled by providing some Bézier curve one end on the left edge of the frame and other on the right edge of the frame. Both ends of the cure have to be on the same level, as left edge is connected with right edge. By moving and deforming the curve user could provide the line of horizon, then by applying effect application should transform equirectangular projection, so the pixels present on user edited curve will be now in straight line in the center of frame.