The "named" cars were reserved for the home market of Japan. Given the company's current marketing strategy, it still wouldn't make a whole lot of sense.
The Mazda Familia is now what we call the Mazda 3 and was the basis for the Familia Rotary, or R100 everywhere else in the world. Similarly, the Grand Familia (higher tier version of the normal Familia) was the RX-3 which later gained the unique name Savanna in Japan. The RX-3 was also the basis of the first gen RX-7, their chassis, suspension, and powertrain are nearly identical.
The Mazda Capella is now what we call the Mazda 6 and was the basis for the RX-2.
The full size/premium/luxury car is a bit more tricky. The 2 door cars have mostly been called Cosmo and the 4 door cars have mostly been called Luce, but there were a few exceptions. The RX-4 was offered with both 2 and 4 door body styles and carried the Luce name in Japan. The Luce R130 was not sold outside of Japan and was only offered in 2 door configuration. It was also the only fwd rotary Mazda ever made.
Interestingly, there was never an RX-6. Rumor suggests that the REPU (rotary engined pickup) was originally to be called the RX-5 and the car that actually was named RX-5 (the Cosmo in Japan) was planned to be called the RX-6. Blame an Australian marketing executive.
The number in the RX series designation doesn't appear to be chronological but rather highly specific to the actual model being produced, at least from the RX-2 onwards. The problem is that most of these models were only made for one generation before the oil crisis happened. There would likely be less confusion if there were several generations of the RX-2, for instance. A fourth generation R100 (RX-1 it probably would have been renamed) could have been this....
1(00): Subcompact car
2: Mid size sedan/coupe
3: Compact sedan/coupe
4: Premium sedan/coupe
5: Pickup (at least it was supposed to be)
6: Premium sedan/personal Grand Tourer (The MX-6 corroborates this)
7: Sports Car
8: 2+2 Sports Car
9: Super car?
Very early rotary Mazdas (R100, R130, L10 Cosmo), RX-5 Cosmo/Repu naming convention oddity and the fact that Mazda launched a failed attempt at a Luxury brand (Eunos) are really the only things that diverged from the pretty straightforward model naming system. If this new car is and built and it is a sports car it should probably be called an RX-7. If it's in a higher class then that, maybe RX-9 could be used.