Hypothesis:
Currently, the stamp brush engine is one of the major ways artists can paint with reusable decals. While this is very useful, there are certain workflows where it can be useful to have the ability to transform and scale the stamp interactively. Furthermore, being able to preview the stamp placement and tweak it in real time before committing to placement could help artists interact with and reuse certain design elements for both animation and game texture creation. The current brush stamp engine lacks these features...

Proposal:
Originally I had proposed the creation of a new brush engine when I brought up this topic during the Krita sprint. However, @rempt is correct in saying that it is not a good idea to implement this behavior as part of the freehand brush tool as it would complicate things. There may be many ways to tackle this issue but a quick idea is to create a stamp tool that can manually place stamps and transform them to fit the users desired orientation. This tool would place a 'ghost' decal at an initial size which could be resized and rotated by moving the mouse. When the mouse button is released it would then commit the ghost preview onto the active layer as painted pixels. This would allow painters to have fine control over stamp placement. It would be nice to support both colorized stamps (stamps with predefined colors for reuse in animation circumstance) and also single-colored alpha-stamps (similar to the brush engine dab.) For reference, both Blender texture painting (DragDot) and Zbrush (DragRect Stroke) support this type of stamping behavior and could be a welcome addition to Krita.