To be clear, the editing notebook is for _files_ it doesn't handle anything else, and changing that is a big change.
Would do you mean by _editing notebook_?
Your other option is to add tabs to the message window, like extra scribble tabs, but that would be a whole new plugin.
It would be also a very different approach I think. Only keeping the untitled files without being forced to save them, that is enough.
To be clear, for contents to be saved between invocations of Geany you are going to have to save them somewhere, and then they will have a unique name, even something useful like scratch-1 scratch-2 etc. so if the plugin does that it should avoid the save query.
I refer to the tab name in Geany. If I open multiple new files, they all will be named `untitled, untitled, ...` without iterator. Independent of keeping their contents after restart, I think it would be a good change if Geany would per default name them `untitled 1, untitled 2, ...` (or capitalized `Untitled 1, Untitled 2, ...`, but I prefer lower case for no reason). We would not need a plugin or setting for that, just a core change. What do you think about that?