Deprecate and remove Activities
It is with a heavy heart that I write this, but the Activities feature suffers from multiple significant issues and I think it should be deprecated in Plasma 5 and deleted in Plasma 6.
It's confusing and doesn't work for common use cases
Activities are difficult to understand, as they have an unclear scope. An Activity contains a private, activity-specific repository of certain but not all sources of config and state data. Because it does not encompass all settings and all state, figuring out what parts of the shell and user apps are Activity-aware is a guessing game. For the apps that do support it, support must be specifically added, which in practice means that only a very small number of apps will ever be Activity-aware, so means the system cannot work generically. Most users use more than just Activity-aware KDE apps. The more this is true, the less Activities work or make sense.
It makes everything it touches buggier
Activities contribute to a combinatorial explosion of options that makes bugs easy to encounter and difficult to triage, especially for things that are Activity-aware, such as desktop containments and panels. Users who use Activities can and do suffer loss of data or important state.
It isn't maintained
The feature seems to be effectively abandoned. I have not seen any substantive work to fix bugs, add new features, or add support for additional apps during the 5 years I have been involved in KDE. Most KDE developers don't use it, aren't familiar with its code, and avoid bug reports about it. The feature is slowly bit-rotting.
With the above in mind, I think Activities' disadvantages outweigh its advantages and we should deprecate it and begin porting away from it, with the goal of removing it in Plasma 6. Features can be migrated back to Virtual Desktops as needed--for example per-virtual-desktop wallpapers.