This is more of a general "big idea" feature request for any EV manufacturer and/or model, but it would be nice if one could buy a minimal size battery pack (say 30kWh), which might be all one needs for most of the year, and then be able to temporarily rent one or two additional packs that could be simply added to the existing pack (a la RAM to an laptop in years gone by), if one needed to take a longer trip. At the end of the trip, the extra weight and rarely-used capacity of the two packs is returned, and a certain fee per day is charged for their use.
This would eliminate the problem of hauling around a lot of unused weight (800 lbs?) most of the time, but still enable long trips to be made using the superchargers, etc. The added packs would need to be nearly identical to the "base" pack, so that their addition would not change the center of gravity, etc. But I don't think that would be a problem for Tesla's engineers if it were designed into the car from the get-go. The "installation" would or should only take a few minutes to do, and all of the software, BMS, sensors, gauges etc. would automatically adjust (again, just like adding RAM), and nothing would be required of the user/owner/driver.
I suspect that there would have been more orders for the 'S40' had this option been available, even though I'm aware that it wasn't quite the same as what I'm suggesting (i.e., the capacity+weight was already there).
So perhaps this is more of an idea for the Model 3. It would keep the base price "way down" (and the miles/kWh up), but buyers wouldn't be missing out on anything if they opted for it. "Sorry" if this isn't as clear as it could be or is covered elsewhere, but it's late and I thought this might be as good a place as any! (Led here from yesterday's CleanTechnica daily email.)