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2011's Cadillac Ciel show car was celebrated for its stylish opulence -- and its high-tech approach: It's a hybrid. GM said at the time that it was a preview of Cadillac's future direction. Source: General Motors.

Let's start with the news: General Motors (NYSE:GM) confirmed last Friday that it will build a new, top-of-the-line Cadillac at its Detroit-Hamtramck factory starting in the fourth quarter of 2015.

Most analysts and reporters immediately assumed this new Cadillac will be the long-awaited "flagship," a new full-size Cadillac sedan set to compete with the Mercedes-Benz S-Class and Audi A8.

That's probably a safe bet. But what if it's not quite the car we're expecting? What if it's not just aimed at Mercedes and Audi -- but also at Tesla Motors (NASDAQ:TSLA)?

This might be more than just a big new sedan
To be clear, some of what follows is purely speculation. But I'll start with what we know for sure -- or at least, what GM has told us so far.

"The objective for this upcoming model is to lift the Cadillac range by entering the elite class of top-level luxury cars," Cadillac's new president, Johan de Nysschen, said in a statement. "Currently in development, this new rear wheel drive-oriented sedan uses completely new, custom-designed materials on a unique vehicle architecture." 

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Last year's Cadillac Elmiraj show car may be a preview of the new top-of-the-line Cadillac sedan's styling. Source: General Motors.

He went on to say that the car includes "new technology" and will require "innovative manufacturing techniques" to assemble.

GM has also said that that the new car will be positioned above the CTS and XTS, Cadillac's current midsize and full-size sedans, and that it will be revealed sometime in the first half of 2015. And we've seen a few spy shots of a heavily camouflaged car that looks like it could be a big Cadillac sedan.

That's what we know for sure. 

Most analysts and observers have assumed that the new car would be powered by the same unique-to-Cadillac 420-horsepower 3.6-liter twin-turbo V6 that appears in the top trim levels of the CTS and XTS.

But what if GM has a slightly different plan in mind? What if the new top-of-the-line Cadillac is a plug-in hybrid?

Why GM might have a big surprise up its sleeve
We know the new Cadillac is a big sedan on a rear-wheel-drive architecture. But it's going to be built at GM's Detroit-Hamtramck plant -- and not at GM's Lansing Grand River plant with the other rear-wheel-drive Cadillacs.

GM's Lansing Grand River Assembly plant is a modern facility, constructed from scratch to build rear-wheel-drive Cadillacs. It opened in 2001, and shortly after, began building the first CTS. It's where the current CTS and ATS are built. It's currently running two shifts; there's room to expand. Adding the new Cadillac to that assembly line would seem to be a natural fit.

But instead, GM is going to Detroit-Hamtramck, or "D-Ham," as insiders call it. D-Ham is one of GM's most flexible factories. Right now, it builds two front-wheel-drive sedans, the Chevrolet Malibu and Impala -- and another, very different, set of products: The Chevy Volt and its siblings.

Earlier this year, GM announced it would spend $384 million on upgrades at D-Ham in order to build the next-generation Chevy Volt "and two future products."

That news came via a GM press release titled, "GM Invests $449 Million for Next Generation Electrification." (The remaining $65 million was an investment in a nearby battery-pack-assembly facility.)

The press release GM put out last Friday had a link to that earlier release and said, "The addition of this new Cadillac is part of a previously announced $384 million investment in [the Detroit-Hamtramck factory]. Tools and equipment unique to the Cadillac model were installed recently."

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The Cadillac ELR coupe uses the current Chevy Volt's technology. Will the upcoming big Cadillac sedan use a version of the new Volt's hybrid system? Source: General Motors.

Read between the lines, and there's a big hint: GM might be bringing some of the next-generation Chevy Volt's technology to this big new Cadillac. 

There are more hints, but we have to go back a few years to see them, to the Cadillac Ciel concept car GM first showed in 2011. When it first showed the Ciel, GM said it would be the first of three concept cars that would show the future direction of Cadillac. (The second was last year's Elmiraj coupe; the third is expected next year.)

The Elmiraj had a V8. But the Ciel was a hybrid, with lithium-ion batteries -- and a 3.6-liter twin-turbo V6.

It could be just what the market wants
As a product, a plug-in hybrid Cadillac super-sedan makes total sense. Combining that twin-turbo V6 with a hybrid drivetrain could make for a compelling top-of-the-line Cadillac: quiet electric power for cruising, with the added power of a 420-horsepower V6 when desired. The result could be a very fast and powerful car that still posts strong fuel-economy results.

Why not go all the way and make it electric-only, like Tesla did? GM may not be convinced that that makes sense for a Cadillac -- at least, not yet. While Tesla's sales have been great, there are still plenty of high-end luxury-car buyers who don't want to worry about finding charging stations when they travel. 

For those buyers, a plug-in hybrid makes more sense. And it could set the stage (and provide some of the underpinnings) for an all-electric Cadillac to follow -- a car that former GM CEO Dan Akerson hinted toward.

But technological innovation is what wins in high-end luxury cars right now. Mercedes' new S-Class is doing very well thanks to a host of high-tech features, including a limited self-driving system. And Audi is also testing self-driving systems and hybrid drivetrains. 

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The latest variant of Mercedes' hot-selling S-Class sedan has a new badge -- and Mercedes' first plug-in hybrid powertrain. Source: Mercedes-Benz.

Mercedes also just launched a plug-in hybrid version of the S-Class that combines a twin-turbo V6 with a hybrid drivetrain. Fully charged, the new Mercedes can get up to 20 miles on electric-only power.

What if Cadillac, drawing on GM's extensive experience with the first-generation Volt, is planning to leapfrog the new Mercedes hybrid -- while giving Tesla something to think about?

I think it makes a lot of sense, and I have a feeling GM's leadership agrees.

What do you think? Is this how GM will go after Tesla? Scroll down to leave a comment and share your thoughts.

John Rosevear owns shares of Ford and General Motors. The Motley Fool recommends Ford, General Motors, and Tesla Motors. The Motley Fool owns shares of Ford and Tesla Motors. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.