I agree, rental auxiliary battery packs should be considered for road trips into "the wild".
A factor little discussed is the COST effect of the Gigafactory. Cell cost will tuble. And 30% lower as I recently read seems highly conservative for what Tesla is doing there.
Not only will the first iteration of the 2170 cells replacing the 18650's improve density from a better steel can to content ratio, it would also likely mean a reducing in WEIGHT for a stuffed battery pack. Oh, and if true, the S and X pack can already accomodate 18700's of they were available. So on top of the ~10% density improvement (starting with the soon to be announced 100kWh 18650 pack), the 2170's would addd another ~8% in just the thickness of the pack used more fully (if true). So a stuffed 2170 pack for the S/X would be ~119kWh right there, when the first 2170's make it to S/X.
And this 119kWh pack may well weigh less than the 2012 85kWH and 2016 90kWh packs. While costing a whole lot LESS due to in-house giga production.
Since people happily pay top dollar for fully packed &X cars to get the best speed and range, a slight update to the S&X chassis might be warrented to fit more cellls still. It could be just a bit thicker, or cooler: an extended chassis. Just add 30cm to the 2nd row leg space, extend pack the same, perhaps thicker where convenient and have 150kWh or so right there. At the same cost as the 2012 85kWh pack, or less, including the extra steel and cabling for the chassis. 150kWh would guarantee absurd acceleration beyond the typical 1/4 mile displays. And, super quick supercharging to 200-250 miles to really get somewhere in a hurry.
The 2012 Model S didn't have a biggger bettery because more couldn't fit the car (IMO), but because the price would become too rediculous. With, being a young company, risk to have to replace it a few times. In 2017, all is different.