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Here I have 2 ideas, i know, but IMHO both are strongly related, so let's keep them together.

The start point is a new feature that may come in with kde 4.3, the (optional) link between virtual desktops (VD) and activities. If I understand correctly this will allow you to have one (and only one) activity per virtual desktop, and then a VD switching will launch automatically an activity switching.

Once you must have one activity per VD (it can be an empty/default one) i think it would be interesting to have a collection of activities. For example you have 4 VD but your collection includes 10 activities, so 6 of them will be unused. We can call them "inactive activities", while the 4 remaining (at most, assuming you don't use the same activity for 2 different VD) are "active". At this point it could be interesting to have an activity-switcher plasmoid, using a combo that contains all your activities (10), including their names, a thumbnail and also a checkbox-like indicator so you can know if anyone is been used or not. With this it would easy to switch the activity linked to the current VD just by selecting it at this combo. Optionally it may allow you to save current config (it's related to the next idea) and/or to set it as the default one (again keep reading ).

Now consider the idea of grouping activities, that is, to create profiles/configs. This should need an activity-profiles-manager. What can you do with it? You can save/load a full desktop configuration, which indicates the activity that is related to each VD. For example, "Config 1" links VD1 with Activity1, VD2 with Activity4, VD3 with Activity6 and VD4 with Activity8. And after saving your set of configs/profiles you also can set the default attribute to one of them, that will indicate the plasma system which one must be loaded at startup. What about having another profile-switching plasmoid, similar to the previous one, so you can change the whole plasma configuration by a few clicks?

If you think about it this profile-capable system makes much more sense once the (in)active activities schema is up, because without it your profiles should only establish the order of your activities, considering that all of them should be "active" full time. And going back this idea of making a difference between active and inactive activities really makes sense if you just can link a limited number of activities to each VD (it will be 1, according to the start point).

Regards

Even if you choose to link activities and virtual desktops (vd), you can have more activities than vd:s. In that regard, it already works as you suggested - if you have 4 vd:s, you can have the same activity on all desktops, or four different activities. All other activities are "inactive", and you can switch to them (on one vd) by zooming out, use keyboard shortcuts or through a plasmoid. More information.

Therefore, I will consider your second idea (activity grouping) as the main idea. As I said in the other thread, this is something that is planned; however, I don't know if it will work the way as your idea.

I think your idea is similar to my idea: [Plasma]Folding and unfolding Activities.
But your is much more clear and set on real-life example instead of vague idea like my.