My money is on a large destination charging push by 2016.
2015 likely will double the total number of SCs, but we should realize that most trips involve not more than one supercharger if you have good charging in both ends. Charging after arriving removes the waiting part, and involves less demand charges (for those paying for the juice to charge) and is ridiculously cheaper than a SC station.
At the same time existing congestioned SCs stations will undergo massive expansion / opening relief superchargers on some areas where there is massive demand. In some areas there will eventually be a supercharger every 40-60 miles, but don't count on that as standard, it will be only in heavily traveled areas.
Why the destination charging push by 2016. At that point Tesla should have 100k cars worldwide on the road, likely with 100k cars production forecast for 2016 (or at least 2017). At that point I doubt there will be a single high end hotel/bnb/resort that won't have received a hundred requests for Tesla charging, once the first nationwide chain closes a deal to get Tesla charging, others will fall in line (like the change from hotel wifi being a rare feature, to everybody must have it). I also expect cheaper hotels to at least provide some 240V charging (even if billed per kWh or per hour). Perhaps 2017 will be the year EVs became truly mainstream (chargers everywhere, solid predictions of a million EVs on the road before 2020). Well one can hope !
BTW, I don't really believe in battery packs beyond 100kWh. The trick is reducing their weight. A 100kWh pack @ current 60kWh weight should deliver 350 miles easily (on a single motor, perhaps 400 miles on a standard dual motor). And a 85kWh pack with weight 20% less than current 60kWh should deliver 300 miles. At those points we're already talking 5+ hours driving @ 65mph. So with a single hour long stop for lunch it should be able to drive 8-10 hours depending on how economic you are. I believe Tesla will find many tricks like the roadster upgrade to further increase Model S range without a battery upgrade.