Elon Musk's Big Tesla Unveiling Could Be Something Totally Unexpected

On Thursday evening, Tesla will unveil the "D" that CEO Elon Musk tweeted about last week. The media event will take place at SpaceX headquarters in Hawthorne, Calif., just south of Downtown Los Angeles.

We'll be live-blogging the event, so check back to find out what "D" actually is.

Until then, let's just keep on speculating. So far, three main theories have emerged:

"D" is an all-wheel-drive version of Tesla's Model S sedan - something Musk has said is coming that the company needs to develop to appeal to customers in the Northeast U.S. and Northern Europe.

"D" is a suite of self-driving or semi-autonomous driving features. Musk has said that Teslas will be able to drive themselves in the next few years, but the technology could be arriving sooner than we think.

"D" is an extended-range version of the Model S - 500 miles on a single charge!

But an interesting new theory is making the rounds ahead of the announcement. "D" could be a renamed Model 3, Tesla's planned mass-market vehicle, which isn't supposed to hit the market until 2017.

This theory is credible because Musk himself said that the Model 3 was supposed to be called the Model E. Ford wouldn't allow Tesla to do that, however. So "E" became "3." Or "III." Not really ideal, given that Tesla's naming convention is based on letters: Model S, Model X (the forthcoming SUV, due in 2015).

From our perspective, it would be great if "D" really is the Model 3. Musk's tweet showed a partially exposed black Tesla vehicle behind a partially open black garage with a giant "D" on the door. If a prototype Model 3 Model D rolls out on Thursday night in L.A., it will be pretty satisfying and rather unexpected. And I suppose you can take a "3" and make it into a "D."

Of course, if something we've seen before rolls out, or if nothing rolls out, it will be disappointing.

At Vanity Fair's new Establishment Summit on Wednesday, Musk said that all the speculation about "D" was "directionally correct" but that the speculators haven't "appreciated the magnitude."

That sounds like something big.