My original prediction was Carlsbad Premier Outlet mall since it has most of the traits other SC's have. Mall, near the freeway, shops, etc.
But as someone pointed out, it's too close to SJC. Encinitas is supposed to have an electric charging stations at an old gas station. But I'm not so sure if it has other amenities Tesla wants. Mission Valley is very central but it's very crowded. Have ever tried to exit Fashion Valley Mall or Mission Valley Center during the holidays? It's crazy.
My new guess would be somewhere near Old Town (a la SJC). It's close to Seaworld, Downtown, shops and restaurants, and people can get to 5, 15, 805, 8, 163 etc. And my second guess would be Kearney Mesa area by old General Dynamics. It's near shops and restaurants and it's still central enough for out of town travelers.