Currently, there’s three different screen sizes for the iPad. The 9.7” 264 ppi display on iPad Air and iPad Air 2, the 7.9” 326 ppi display on iPad mini 2 and 4, and the 12.9” 264 ppi display on iPad Pro. iPad mini was introduced just two years after the introduction of the original iPad, iPad Pro was introduced just three years after that.
If I had to take a guess at what the next iPad would look like in two or three years from now, I think it would be something that’s pretty obvious from the screen sizes listed above.
The iPad mini was a scaling of the iPad’s ppi so that it was the same as the iPhone. I think the next iPad will be the same scaling again from 264 ppi to 326 ppi, but from iPad Pro rather than from iPad.
So that’s a 10.47” 326 ppi display at 2732px by 2048px. But Apple wouldn’t just make a new iPad with this screen size because it solves a fun math problem. I think the screen size and pixel density actually make a lot of sense. For the person that wants to replace their computer with an iPad, they aren’t doing it so that they can have a better notebook computer. They’re doing it so they can use a great tablet. iPad Pro is too unwieldy for most people to use as a regular tablet. At the same time, the iPad Air doesn’t feel as powerful as the Pro because you simply can’t fill the screen with content like it’s a notebook computer.
I think the device would be large enough to warrant the technology advances that were brought to iPad Pro - the Smart Connector, Apple Pencil, etc. It would also bring the inherent ergonomic advantages - more roomy canvas for drawing, comfortable on-screen keyboard, etc. This iPad could optionally use Display Zoom to render the display at a 264 ppi equivalent. That would mean that the device wouldn't necessarily ignore people who like the interface sizing of the less pixel dense iPads. For the person who wants a powerful tablet, but doesn't want to break their wrists or the bank, this could be a great product. In many ways it would be an iPad Pro mini. But I think Apple should just call it iPad.
Update
Apple announced a 10.5" iPad Pro on June 5, 2017. I was off by .03", 62 ppi, and the name of the device, which ended up going to their low-end 9.7" iPad.