Well, first of all AppImageKit is not a package manager. For a comparison with xdg-app and other systems, see https://github.com/probonopd/AppImageKit/wiki/Similar-projec...
As for sandboxing, this is definitely an area which we would like to add to AppImageKit, e.g., see https://github.com/probonopd/AppImageKit/issues/77 - thoughts and pull requests welcome!