[UPDATE 2020.09.23] live demo (just for illustration)

WHY?

• In some cases, the animator is mostly interested in arc and ease-in, ease-out. With the "motion path tool", we can view motion paths, velocity changes, and edit them either, right away in the 3d viewport without any caring about graph editor with other complicated curves. ( three curves (x,y,z) can be replaced by one curve ).
• Blender's current motion path looks like a tracking path. This means it only shows the trajectory of objects or bone as an attribute. When the motion path is active, it is always visible even if the user is not using Blender for animation (in edit mode, sculpt mode, etc.). And sometimes, we want to show only the motion paths of certain objects, the current way is to disable the motion paths of all other objects.

PROPOSALS

I suggest adding motion path tools in object mode and pose mode.

• motion path is visible only to the selected object or bone. If the user wants to display multiple motion paths, just select multiple objects.
• Users can manipulate the motion path gizmo like other tools. Control points and handles to change position and trajectory. Easing handle to change acceleration, and so on.
• With the Motion path tool and own tool settings, we can easily add advanced modes, like visual editing options for either rotation or scale.

HOW?

MOTION PATH

During the study of the relationship between motion path and F-curves. I found that: if I bind F-curve handles of x and y channels together so that they have the same time value ( X direction), I could easily create the Motion path (MP) with 2d coordinates of the control points and handles based on the value ( Y direction) of the keyframes and handles of x, y channel (Fcurves) :

• MP control point [x, y] = keyframe [x_Fcurve.y, y_Fcurve.y]
• MP handle [x, y] = keyframe handle [x_Fcurve.y, y_Fcurve.y]
• x_Fcurve.x = y_Fcurve.x

As a result, the motion path matches the location tracking point.z

TIMING MANIPULATION

Furthermore, I can change the easing interpolations (by only shifting the F-curve handles horizontally) right away on the motion path through the easing handle. The gif above shows us how intuitive when the Easing handle and tracking point shift together along the motion path.