For many years I’ve used Ksnip to take many screenshots in a row to annotate later. It’s a very useful feature that spectacle doesn’t have.
Considering that Wayland is slowly gaining ground and that Ksnip doesn’t seem to be able to support rectangular regions on Wayland (development seems to have stopped), Spectacle could gain a lot of users (like me) if it incorporated this option. I would do it myself, but I don’t have enough knowledge.
you can save screenshots to a folder annotate them later using spectacle -E
i’ve set up a separate .desktop file for the -E option and i can use open with… from the context menu to annotate and save screen shots that i’ve already taken.