Animate All (AnimAll) for Grease Pencil vertices

Just like animating with AnimAll for object meshes, lattices, etc... give Grease Pencil vertices / strokes the same ability.

Example: In frame 1 a drawing of a square. In Edit Mode, grab desired verts and strokes.
With verts / strokes selected, use "AnimAll > insert keyframe" Copy selected Go to a new spot on timeline, say 5 seconds away. Past the copied verts and strokes to create a new frame. Translate, rotate and scale as needed. In Sculpt mode: Push the same verts and strokes around to a new shape and position. With desired verts / strokes selected (in Draw, Edit or Sculpt mode) use "AnimAll > insert keyframe".

Playback animation and watch the morph from frame 1 to the next frame 5 seconds away. There would be no other frames added between these (unless something is manually drawn, of course).