Elon said it during the last earnings call, we really should find a better name for the GenIII car. Here it goes then. Assuming we are staying withe the "Model -" type naming, I think it might make sense to call the GenIII line of cars the Model M.
We can then have variants:
Model MS = Sedan
Model MX = Crossover
Model MT = Truck
The question would be the next Roadster which I will guess gets it's own letter in Model R