In getting back to the original poster's question of what will potentially be Tesla's next big thing to propel it beyond the existing business case fo selling cars, I can think of a couple of different ideas to keep Tesla going to the next big thing. The firt idea I have is that I think it's going to be in terms of what to do with all of this excess battery capacity in the future. I do completely agree with Sleepyhead above though about building their own battery factory as increasing margins and decreasing COGS, but I'm talking about all of the batteries in 6-8-10 years from now as people trade in their Model S or X and Tesla upgrades the battery to the latest tech to sell it on the used market as their CPO S or X Sales. I think they are going to have potential for a huge supply of slightly used 50-70kwh remaining capacity battery packs in suprlus looking for applications.
I think the market could be huge to use these as local power storage units, residential or otherwise, especially combined with Solar City. THere would really be no reason at all for someone to need a home backup power generator if you've got some solar panels and a couple of monster batteries ready for it. I see this capability being huge for someone like SOlar City whose business model seems built on energy arbitrage to/from the power company. Once the power company changes the regulations to charge more for maintaining the lines and less for the generation of power, then the need for home power stoarge will be critical. And there's Tesla with this huge market for batteries more than qualified to do the trick that have essentially already been paid for and now able to be sold again.
The second growth vector I see along similar lines is the peaker power demand model for the larger supercharger sites. Being able to have enough power and capacity for grid storage to help the power companies reduce the need to maintian the peaker power plants currently maintained for that very small level of demand when the power needs are highest, could ultimately prove to be a potentially large income source in my mind for Tesla.
The next thing I see which hasn't been mentioned in a while is the SC network itself. I think licensing that technology to another major automanufacturer once it is compeletely rolled out and operational has potential again to be as much of a moneymaker as their primary business of making and selling cars.
I think the market potential in all three of these ancillary growth vectors are enough to keep me extremely bullish on Tesla's prospects as a company. I'm not a Teslanair just yet, but by maintaining my long term bullish stance through their announced plans for making and selling the greatest cars in the world, and my optimistic belief in the above, I hope to be joining that club sooner rather than later. I've been very bullish and a stockholder since 2011.
Thanks,
Wayne