Currently, as I see it, Activities is mostly a pointless gimmick feature (no offence) in Plasma if you use non-KDE applications (both because non-KDE Apps don't get relaunched the intended way when starting up shut-down Activities and because it's barely mentioned as being a feature), and it'd be a whole lot better being partly merged into the workspaces feature (with the non-merged stuff going bye-bye permanently). Plus, if it means features like monitor wallpaper spanning officially being in Plasma finally, it'd make so many people happy, sacrifice aside for that.
That tangent aside, Activities have a bunch of good features that I don't want to see removed, but rather merged into places they'd work better in:
1. Move the desktop background changing to be a feature across different virtual desktops instead of across different activities (and therefore different monitors) - this could work hand-in-hand with https://phabricator.kde.org/T12620
2. Allow all the virtual desktops to have the application session saving functionality of Activities, when Desktop Session -> Session Restoring on logging into Plasma is turned on. (if it doesn't already behave this way)
3 (silly idea that could be pretty cool to have). Allow workspaces to be 'Set Aside', akin to Classic Microsoft Edge's feature of the same name (this would essentially be like shutting down an Activity and would allow people to keep certain workspaces out of the way for later use)?