The Model S is so good that all they need to do is make it into a SUV (Model X) and they'll have the coolest high-end SUV (coming 2014).
Then, they just need to make the Model S smaller (GenIII Bluestar) and then they'll have the best small luxury sport sedan on the market (ie., BMW 3 series, Lexus IS, Audi A4 market). Elon Musk has said they're going after the BMW 3 Series market with the GenIII car, and he's determined to make a best car in that market. Just like the Model S is better than a BMW 5 Series in most regards, the GenIII will be better than the BMW 3 Series in most regards. I know it might be difficult for many people to accept that (especially since the BMW 3 Series is legendary) but Tesla's got all the right pieces and has proven they can do it with the Model S. Again, they just need to shrink the Model S and make it more affordable.
But GenIII is slated for 2015 at the earliest and it takes a while to dominate a car segment even after you have a stellar car. I expect Tesla to become increasingly competitive (in terms of # sales) to the BMW 3 series by 2020 and perhaps dominant by 2025 (if not earlier).
Elon Musk has repeatedly said that Tesla Motors is not a typical car company but rather a technology company that will innovate at a blistering pace. As long as they continue to do that, they'll be fine.