TL;DR: Plasma does not need both virtual desktops and activities (as a more evolved alternative). Instead both concepts should have clearly defined and separate roles: desktops for window management and activities for semantic desktop (only). In this setting, it should be possible to semantically associate a virtual desktop to an activity (as you would associate a document).

First, I would like to apologize, as I do not know whether here is the place to make such an elaborate suggestion, but I do not really know where to start either... (probably I should directly discuss with whichever developer is currently interested in this part of plasma, but I am not familiar enough with the project... ). Also maybe this issue should not be reported to kactivitymanagerd component, as it is a transversal one (please feel free to move it to wherever it should be discussed!).

So I believe current situation with respect to activities and virtual desktop is less than ideal (anyway I am not very satisfied!).

On one side, we have virtual desktops:

- old concept, shared by many X11 (and now Wayland) window managers
- each window has, in its properties, the desktop number it belongs to
- various apps from different desktop environments are desktop-aware in a compatible way
- they are well supported in plasma/kwin, with nice switching effect and convenient integration (window title bar context menu, task manager and virtual desktop plamoid)
- but overall they are quite poor in features

On the other side, there are the plasma activities:

- like virtual desktops, they can contain windows...
- ... but they also contain associated documents and various stuff. Power management is also activity aware.
- newer concept (although not that new now!)
- kde plasma-centric. Non-plasma apps are totally oblivious to the concept (when you restore your firefox windows after it crashed or after you rebooted, they all appear in current activity disregarding where the windows used to be before, for instance).
- even kde stuff does not fully support activities everywhere it makes sense (various reported issues)
- features similar to virtual desktops, needs to get re-implemented separately (for instance the activity manager plasmoid, analogous to the desktop manager plasmoid)... sadly often it is done in a less polished way (many kwin animations for desktop switching, none for activity switching) or not at all (kwin has many shortcuts for navigating desktops or presenting windows from current desktop, but not their counterpart for activities).

While it appears activities were initially meant to replace virtual desktops, after all these years, we are not yet at a stage where they can efficiently do so. Even if many issues could be fixed, given some love, there would still remain the main standing issue with the non-kde world.

As a consequence, if you want everything (full support and semantic desktop features), currently you need to use both activities and virtual concepts. Unfortunately, doing so does not resolve everything and it adds a useless layer of complexity, which is hard to mentally adapt to. Also this adds constraints that do not make sense semantically (such as being forced to use the same number of desktops in each activity; or, if we look at it with the opposite mental model, being forced to have the same activities on each desktop!).

Moreover as long as plasma uses both concepts, many features need to be duplicated which wastes a lot of developer time.

At this point, wouldn't the only reasonable thing to totally drop window management from activities and only keep the semantic stuff?
What I have in mind is that an activity should not be a kind of virtual desktop on steroids, but just container for :

- as it is already the case: documents, files, folders, tasks and some relevant settings (e.g. power management settings)... and not windows!
- but also a set of virtual desktops (so that every virtual desktop is associated to some activity)

When you switch to a desktop, you also automatically switch to the activity it belongs to, hence:

- all the activity switching and window management features can be removed (activity manager plasmoid, keyboard shortcuts) so that their (often more polished) virtual desktop counterparts can be used instead;
- non-kde apps who know how to deal with virtual desktops (remembering where they belong, for instance), also work smoothly with activities (activity membership entails from virtual desktop membership).

Would that make sense?