In ksnapshot existed the option to take a snapshot of a "Section of window": A portion of an active window. Why isn't this part of Spectacle? I would appreciate the integration of such a productive feature.
Have you tried taking a "section of a window" screenshot of a modern app with KSnapshot recently? e.g., a Qt5 app? It doesn't work anymore. That functionality relied on the fact that in the old days every widget in an app ("section" of the window) was its own X11 subwindow and could be targeted individually for capture. This is no longer the case with modern GUI toolkits - the only "section" in modern apps cover the whole window.