I think DL should add at least three more boarding groups before Zone 1, just to make it more confusing.
Pre-boards
F
DM
PM
GM
FO
Zone 0 (for Kettles)
then
Z 1-3.
Seriously though, DL should just use Zones 1-5 for everyone. Zone 1 is F class, Z2 is Sky, Z3 (old Z1), etc. Today's system is so confusing when pax in Z1 see half of the plane get in line before them, they all start wrestling into line thinking they missed the Z1 call. Make it simple.