I expect -- and hope -- that Tesla is being very accurate when they talk about competing with the BMW 3-series. Having a sedan, coupe, and convertible all built around the same chassis and drivetrain, offering four seats and a reasonable amount of space, for $30K to $60K (after tax credit) is BMW's bread and butter. Those cars sell like hotcakes... and I still miss my 335 convertible.
The Gen-3 car MUST have a four-door (or five-door) option... that's the standard model to generate the most sales. Coupe and convertible alternatives will broaden the appeal of the platform to different buyers and create more potential customers while maintaining very high parts commonality. Having two -- or ideally three -- battery capacities available would make this car sell extremely well. Other things that would be very welcome IMHO are the Performance option, an AWD option with dual motors, and an AWD Performance option. Tires should likely be 15" with an option for 16"... but go with whatever the market wants as long as the Performance version can get some really good rubber to maintain best-in-class handling. All of those -- sedan, coupe, and convertible -- can be part of the same "Model C" or whatever they choose to call it. Easy to say you got a "C sedan" or a "C convertible".
For my own personal car, I'd like the "C convertible" with four seats and good trunk space. I'd get the AWD Performance version, which hopefully would outrun the Model S by a little bit.
As I noted recently on another thread, I would also create a supercar "Model R" based on the same small chassis. Make it also AWD with two motors from the Performance S, and you have 830 HP and an unholy 890 lb-ft of torque. Two-seat, drop-dead-gorgeous body that'll attract as much attention as the Roadster currently does (I've had Lamborghini drivers stop to stare...
), use the largest battery available on that chassis as standard, and Tesla has a new flagship they can sell for $100K.
With the Model S and Model X sharing one chassis, and the Model C and Model R sharing another, Tesla will have a very attractive and functional product line. Other vehicles are possible, but I'd sure start with those.