I imagine it as some kind of GUI with “boxes”, each defining a “workspace” with specified permissions (access to that and that folder, network, etc), such that the user could easily drag and drop apps into new workspaces.
This way, I could have a “banking” workspace just containing, say, firefox, and a “work” workspace with, say, firefox and thunderbird and whatever, etc. The “workspaces” would by default be as unprivileged as possible, with the possibility to give access to folders, or even maybe to use unionfs or similar to combine the views of different workspaces.
For ease of use, I could assign a color or icon to each workspace, and the manager would automatically generate desktop files so that I would easily be able to spot “banking firefox” from “work firefox”.
I feel like with bubblewrap, this is only a short python GUI away, and would be super helpful to strengthen security on linux, even more so for non-technically inclined, or lazy people.
[Edit:] If someone wants to discuss things further and even maybe try and write a “MVP”, I'd be happy to try.