We don't really have to speculate that much. Following Elon closely throughout the years, it's obvious that he never lets his ideas go, he just adapts them to the real world (meaning his projects get postponed a lot, but he always stays true to his vision).
The next generation Tesla Roadster is coming, however it will not arrive before 2020 or later. (Musk said 2019/2020, so I added Elon-time). Most likely it will look similar to the first Roadster but with perfected performance and range with the technologies available at the time of design/production (= lower drag coefficient, better battery, drivetrain and software). The whole "after Ludicrous mode comes maximum plaid" -joke was made by Elon in reference to the next-gen Roadster.
Only since the reveal of the Model 3 has Elon spoken about the model Y, as someone posted here before me. It is coming, and it will be a model 3 variant, similar to what the Model X is to the Model S. So expect a model 3 that's a little more bulky with more room and falcon wing doors. (See the tweet by Elon where he confirms model 3 and Y, and he states that only one of them will have falcon wing doors. We know for a fact that this isn't the 3, so it's the Y).
The true question is, what's after the Model S, X, 3, Y, next-gen Roadster and the possible (but not really confirmed by Elon, only speculated about by him) pickup truck model P?
An overhaul of all those models in order of original release is unlikely IMHO, because Elon also hinted that he doesn't want Tesla to have "Build models" (like say a BMW 5-series of 2008-2011 for example, which has a particular styling and performance). He stated no real drastic change would come to any model, he would just keep improving all cars at all times. So the best time to buy one is always "now".
Honestly I think Elon will move his attention towards SpaceX fully after all the above models have been released. By that time EV's should have become quite standard, given the competition will also flood the market with EV's. So his goals with TSLA will have been accomplished. Hopefully TSLA won't act like Apple when that time comes, and start to ride on previous success...