I'd love to have this, but I see two near insurmountable problems for most teams:

- velocity metrics go down and timelines stretch, because effort is "wasted" on things that don't get finished.

- someone needs to do the boring hard work of laying out a whole project in advanced, fairly clearly if folks are allowed to jump around on the project.

Without those two, either the team looks bad to management or folks get stuck in a dead end even if they want to work on something (or spend their whole "20%" time just gathering requirements)