The shifting sands in relation to designation is actually adjustments rather than adding new models -
The UKL based Z1 will allow a straight forward BMW entry Roadster (similar to the MINI Roadster) and that car is a lot of fun. a real go-kart.
The Z3 will be the original Z2 proposal - , based on the F20 matrix be lighter and offer a Soft-top and a Coupe like the original Z3. a proper Roadster.
The Z5 will be the natural progression of the Z4 , allowing it to retain it's current premium image and folding hard-top to remain a Coupe or Roadster.