I noticed that there is feature duplication between Gwenview and Spectacle. It seems there are 2 codebases doing the same thing, and spectacle's is better in my opinion. There even is a dolphin context menu "make notes with spectacle" just to launch this edit mode. Why not split it into a separate package, to use it from within dolphin and gwenview too? That would be pretty nice I think!
Yes, during development of the new version of spectacle, turning the new annotation system into reusable components was brought up as something desirable. It would probably need more cleaning up in order to turn it into a library with stable API. Before it is made into a library with stable API, I think that at least one other app should implement a UI using the components. Preferably two, but there might not be enough apps that need these components.
Koko would be an obvious candidate, given that it offers similar functionality to Gwenview and has a QML UI.