@bepolymathe yep, so i think the use-case/workflow is quite common, but Joplin has not quite got the functionality (yet!) - though to be fair, very few note taking systems do! Evernote has saved searches, which is close'ish - the Favorites plugin offers the same, but the search function isn't working atm (fingers crossed). On the same topic though, Evernote doesn't allow a deep notebook hierarchy either, so there's that!
ideally what I would like to see is the Notebooks tree as it is today, but Notebooks are either "smart" or "fixed/static" (of course I would love hybrid as well... but baby steps) - and this is what I mean by Virtual/Dynamic/Smart Notebooks - they look and behave as Notebooks, but are updated automatically based on specific criteria.
Someone who knows the internals better than I can answer, but would it be difficult to achieve? A plugin maybe, but it feels like something better as core integrated with Notebooks.
I am thinking Saved Favourites a temporary workaround and hope it comes sometimes in the future 
