My thoughts are :-
Tool nose radius comp needs major work.
X axis threading.
Offset table added in as a screen tool not an external window.
U & W added as incremental moves.
I,K & C added as automatic chamfer.
R added as automatic radius
G92 added to threading options.
G76 to use standard Fanuc 11 format as an option. E.g. G76 X(Root Dia) Z(End Point) I(Taper) K(Depth) D(Depth 1st Cut) F(Lead) P(Style Flank, ZigZag Etc.)
G50 to cap speed in CSS mode e.g. G50 S2000
GOTO, WHILE, IF Etc. in g-code.
That will do for starters, I am sure there are more.
Graham