For months, there have been rumblings throughout Apple's supply chain that the company is working on a new iPad with a larger screen. On Tuesday, those mushy rumors got a bit more credible when Bloomberg reported that Apple plans to launch a larger iPad with a 12.9-inch screen in early 2015.

For reference, the iPad Air has a 9.7-inch screen and the iPad Mini has a 7.85-inch screen. The top model of the MacBook Air has a 13-inch screen. So this new iPad will be a massive tablet, and something you likely wouldn't want to lug around everywhere you go like you can with a smartphone or the superthin iPad Air.

Others have tried making supersize tablets before. Toshiba's Android-powered Excite tablet, which came out in 2012, had a 13-inch screen. It was simply too big:

The 13-inch Toshiba Excite tablet, which launched in 2012. It was huge. Steve Kovach, Business Insider

So, why would Apple want to create a giant iPad?

Let's do some sleuthing.

iPad sales are in decline. In the second quarter of this year, iPad unit sales were down 9% on a year-over-year basis, a trend that Apple seems to be having trouble turning around. This chart from Business Insider Intelligence sums it up pretty well:

There are several theories for why this is happening. Some people think the iPad upgrade cycle isn't as frequent as it is for iPhones. Whereas you may typically upgrade to a new iPhone every other year when your carrier contract is over, some think consumers are keeping iPads for three or four years before upgrading.

Another popular theory is that many people are realizing they don't even need an iPad in the first place, considering the MacBook/iPhone combination is more than adequate to get it all done. The MacBook is for productivity at your desk, while the iPhone helps you on the go with some fun apps, games, and social networks thrown in. The iPad is just a big iPhone, the theory goes, and therefore doesn't really justify the extra $400 or more you'd have to pay to use it.

It's the second theory that sounds like the most plausible culprit for the iPad's slump, and it's part of the reason why we've seen so many attempts recently at hybrid devices that try to merge the tablet with a laptop.

Microsoft's Surface Pro 3 is the best example of that. Since the device was first introduced, Microsoft has marketed the Surface Pro 3 as "the tablet that can replace your laptop." Snap in the special keyboard cover, and you have a full-featured PC. Snap it out, and you have a regular tablet for kicking back and having fun.

The Surface Pro 3 tablet doubles as a full-featured laptop. Steve Kovach/Business Insider

That's the promise, at least. But as just about every review of the Surface Pro 3 has said, the device fails to live up to that promise. The problem with the Surface is that it requires you to flip between two radically different interfaces: a traditional desktop mode, and a touch-enabled "modern" interface. It's clunky and confusing. Yes, the Surface Pro 3 inches us closer to that dreamy device that can do it all, but we're not there yet.

But that could be what Apple has in store for the 12.9-inch iPad. (Let's just call it the iPad Pro moving forward.) Think of it as a laptop that reimagines what laptops should be able to do. A "hybrid" like the Windows 8 devices out there wouldn't be the best description because Apple would likely never load two different interfaces onto one device.

What's more likely is that Apple implements a multitasking feature into its mobile operating system, iOS. In fact, we already know iOS 8, the next version of iOS, will allow such multitasking on the iPad. According to 9to5Mac's Mark Gurman, Apple will update iOS 8 a few months after its fall launch to allow split-screen multitasking. Others have found hidden code in iOS that shows Apple is experimenting with split-screen apps.

It's very similar to the way Windows 8 lets you run two touchscreen apps side by side. It could look something like this:

Throw in a clever keyboard cover, and you may be onto something.

Then there's the whole productivity problem. The common theme around iPads is that they're only good for the fun stuff like social networking and watching videos.

But the apps are getting better. Microsoft finally released Office for the iPad this spring, and it's really good. Apple has its own suite of office apps that come free with every iOS device. Google Docs on iOS now come as separate apps and are compatible with Microsoft Office files. Other startups like Quip are completely re-imagining what it means to get stuff done on a tablet.

Plus, more big companies will likely be snapping up iPads thanks to Apple's partnership with IBM. IBM will use its salesforce to sell iPads to its big enterprise customers and provide a lot of business apps with them.

Apple CEO Tim Cook himself recently told The Wall Street Journal that he's able to do 80% of his job on an iPad. In an interview with Re/code's Walt Mossberg on Tuesday, Cook called the disappointing iPad sales a "speed bump," implying that things will turn around.

With all the talk about Apple breaking into new categories (TV, smartwatches, and big-screen phones), it sounds like it's also gearing up to reinvent the tablet as a productivity tool. Apple could very well be working on the dream device we've been asking for, the best of a laptop and tablet in one.