Hello! Actually, a fun project would be to implement Nix storage model, its build primitive (‘derivation’), and assorted tools (garbage collector, etc.) in Guile Scheme. It doesn’t seem unrealistic, though it’s perhaps another project; OTOH you mentioned rollback so it’s not completely irrelevant either. ;-)
Another (difficult) question is how that would integrate with the rest of GNU—the GNU Build System, GSRC, ELPA, Stowfs [0], etc. Thanks, Ludo’. [0] http://www.gnu.org/software/hurd/hurd/translator/unionfs.html