Consider this nomenclature that would be consistent with all but one of the existing models.
Odd numbers for all chassis that are available in 4-door form.
Even numbers for all chassis that are ONLY available in 2-door form.
The existing even number models are the Z4 and 6 Series, both of which are ONLY available in coupe/convertible form.
However, the existing odd number models are X3, X5, 1, 3, 5 and 7 Series and they are all available, but not exclusively, in sedan, hatchback or SUV form.
The only exception to this rule is the X6, but one could argue that it is a coupe-like SUV.
So with the introduction of a 2 and 4 Series, BMW could make these models coupe/convertible-ONLY models, like the 6 Series is to the 5 Series. The 2 and 4 Series could be loosely based on the 1 and 3 Series chassis respectively, but it would have an entirely unique body work and could blend in features from the higher models (3 and 5 Series) to bridge the gap, again much like the 6 Series does with the 5 and 7 Series. So I think the 2, 4 and 6 Series should be dedicated coupe/convertibles that are loosely based on the 1, 3 and 5 Series respectively but command a higher price tag due to the body style and greater features.
That way the M3 can stay M3 Coupe and that allows BMW to bring back a M3 Sedan whenever they want. The new 2 and 4 Series would not be a simple 1 or 3 Series with two doors and a roof chopped off. It would be its own model like a baby 6 Series.