A better way to frame it instead of one user per project, is to have one persona (casual me, working me, planning me) per workspace, each of which can only be used by one identity on the whole machine.
Multiple desktop shell sessions per user. Oh yeah, that would rock
The ability to switch to a project, and see all my notes, documents, apps for only that project. In a way that doesn't collide/interfere.
maybe have ~/Desktop/project1, and all apps use that to store settings, notes, etc.
could be a group of desktops for a project.
maybe like virtual desktops for a project in columns, but each row is a different a separate project.
The idea would be close to macos with fast user switching, but one user per project.