You can't necessarily use the pixel densities and design considerations of the iPhones or even the iPad mini to determine the pixel densities of the 10" iPad Air. The design considerations are different, as are the usage patterns. 325-350 ppi is necessary in a phone because we hold them in front of our face just inches away. Like 10 inches away. I don't think I've ever met anyone who uses a laptop this way (although I've seen people who are near blind use desktops up close). Indeed, for the iPad, people don't generally hold it that close either, although kids with the mini might hold it a bit closer.
With the iPhone 6 and 6 plus, the reason to go to 1080p on the 6 plus is due to the OS design. It allows 3X pixel densities to maintain reasonably constant element sizes, sort of. John Gruber theorized the 6 plus would actually be higher than 1080p, at 2208x1242, because mathematically, that would allow element sizes at that screen size that are only 6% magnified from what they are on the iPhone 5s. In fact, if you do the math, or if you just measure it, you'll realize that the element sizes for all the iPhones from the original to the iPhone 6 are all exactly the same. The only one that is different is the 5.5" 6 plus, 1.06% normal. But why are we talking about 2208x1242 then, if the 6 plus is actually 1080p? Because John Gruber was actually right, in a sense. All the calculations on the iPhone 6 plus are actually done at 2208x1242. Then it's actually downsampled to 1080p for display. Presumably this is just a decision of practicality. 1080p is a common 5.5" screen size now, so the tech is already available in volume. Furthermore, at such high pixel densities, there is no visual penalty for downsampling. 2208x1242 still looks gorgeous when downsampled to 1080p. In contrast, the iPhone 6's 1334x750 would look horrible at 5.5" on the 6 plus. So, they jumped up to 2208x1242 for the 6 plus, but then downsampled it to the also high 1080p.
On an iPad Air, just about nobody complains about the pixel density, except for occasional spec weenies. It already looks gorgeous at 264 ppi, which BTW, is way higher than the Retina MacBook Pros, at 220 ppi on the 15" and 227 ppi on the 13". There is essentially no need whatsoever to go to 3X 3072x2304. Furthermore, with 2048x1536, Apple can design its SoCs to work with both the iPad Air and the iPhone plus at 2204x1242, which translates into 3.1 MP and 2.7 MP respectively. If they went to 3X 3072x2304, the Air would be a huge outlier, and either they have to have a much higher spec'd common GPU for all iDevices but which would be overkill for all the other iDevices, or else they'd have a separate A8X just for the iPad Air... sort of like the iPad 3 days. However, in this scenario, there is just no need for that outlier. The A8 powers the iPad Air, iPad mini Retina, and 6 plus no problem. I'll reiterate since it's an important here: Despite how high the 6 plus's internal rendering rez seems, it's still significantly lower than the rez of the iPad Air, so an A8 powering a 2204x1242 iPhone 6 plus will have no problems powering a 2048x1536 iPad Air 2, esp. since it will probably slightly higher clocked in the Air.
The iPad mini Retina has a higher pixel density, but it also has the same 2048x1536, which means that SoC design considerations don't have to include any resolution higher than the flagship iPad Air. And all of those screens look gorgeous in terms of detail. (The iPad mini Retina isn't as colour accurate however, among other things.)
There is one thing I have not talked about though, and that is the rumoured 12" iPad.
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