Well, the browser already did replace Unix for most people=( Presumably we'd be trying to design something good and not awful though, so lets not stop there. But it does give us a starting point:

+ one click installation of applications

+ applications are always up to date

+ applications are sandboxed

Then, since we want users to be able to tinker:

+ the source for all software installed through this mechanism is available

+ optional binary caches NixOS style so it's not slow

And while we're talking about NixOS:

+ immutable package directory so you can rollback at will

And then on to data storage, which is my favorite topic:

+ defaults to tags instead of a hierarchy for organization

+ everything is schema'd, so a file saying it's a contact entry or whatever actually has to be that thing

+ data is immutable by default, so undo and versioning can be built-in (you could still turn this off for certain tasks like video-editing where immutability isn't feasible)

I have more thoughts on data storage here if anyone's interested: https://juniorschematics.com/