A car for all seasons has to have certain qualities, not all of which are covered by the 911.
It has to have good handling and braking, probably enhanced by some judicious modification.
It should be a closed coupe (stiffer, and all weather).
It should have good power output. It should not be a gas guzzler.
That last one is an issue. Power is easy to get, but when you do it by tuning conventionally you inevitably make trade offs in fuel consumption and drivability, as anyone that has ever driven a race tune car in heavy traffic can attest to. Fortunately there is a ready answer - turbocharging. You can get excellent mileage while just puttering around, and then when you hit the accelerator, it is like calling a few extra cylinders on line. There is only one other way to get that combination of flexibility, economy, and power on demand, and that is VTEC, most capably implemented by Honda.
I've got several cars that would make decent dual purpose cars, but many fail on one criterion or another.
The Jensen CV-8 handles decently and unusually for that period has 4 wheel disc brakes. The big block Chrysler with sixpack puts it out of the running on economy, though. Same with the heavier Interceptor.
My Lamborghini is a street car but it was built by the factory like it was a race car, so it is a dream to drive hard, but with 6 Webers sucking away, it gets the fuel mileage of a drain.
The MGA coupe would be quite good. Four wheel discs, decent mileage with stock early MGB engine, and modded suspension to give very good handling with the wider wheels and modern rubber (period high aspect tires look nice in shows, but don't come in performance rubber compounds). Power might be low for some, though.
The Jamaican bodied MGA would be even better - 2000 lbs. with 200 or so BHP (net, not gross) to work with, a modern 5 speed, runs on 87 octane with a long 5th gear for relaxed low consumption touring.
My 88 Fiero - pretty good. Good handling (also modded), around 300 BHP from a modified turbo engine, but not so good mileage in town (pretty good on the highway).
Best of all would be my 09 Pontiac Soltice coupe. It has better handling by far than the early 911's do (yup - modded again) and with 350 BHP and 1/4 mile slipping into the 12 second bracket will blow away all but the most recent Porsche or some of the earlier turbo Porshces. The magic is that it gets this from an efficient (direct injection) 2.0 Ecotec engine that gets more than 30 mpg when you keep your foot out of it. This one is my favourite all round sports car, and I find it ironic that it is American, considering the generally pathetic level of American built cars in terms of traditional ideals of handling + performance.