And your prediction is wrong. Apple will harmonize the Retina HD on all models since the new screen still has the same resolution of 326 dpi, but different pixels (dual domains) with glass lamination to screen.
iPad Air and Mini will have same screen off course diff sizes, same internal specs and $100 price difference.
Apple may evaluate the Mini one more year before ditching it in near future...1-2 years from now...probably.
Agreed, Apple will not fragment the Air into two different display resolutions. What a mess that would be!
It looks like they may indeed end up with different resolutions on the Retina Air and Retina Mini. There's the iOS 8 reference to 3x display iPad scaling, which is believable for the Air:
Original (1X): 768x1024, 132 ppi.
Retina (2X): 1536x2048, 264ppi
Air 2 Retina HD (3X): 2304x3072, 396 ppi
I wouldn't be surprised if we see another implementation of resolution downscaling so Apple doesn't have to spring for ~400ppi at 9.7".
But the Retina Mini?
Retina (2X): 1536x2048, 326 ppi
Retina HD (3X): 2304x3072,
486 ppi?
No way does Apple use 486 ppi, that is well into the density range with terrible efficiency and little visible advantage. I'd guess the Mini remains at the current resolution but get's a laminated LTPS display with the groovy pixels. So maybe we get an iPad split similar to the iPhone split: small one at 1536x2048, 326 ppi, large at 2304x3072, ~400 ppi, both using the same LTPS and skewed pixel tech for gorgeous image.
BTW, no need to calculate PPI by hand via pythagorean formula, just use
this calculator.
Edit: On further thought, the 3X resolution makes the most sense for the mythical 12.9" iPad Pro:
iPad Pro 12.9" (3X): 2304x3072, 298 ppi
As much as I'd like to see the Air 2 get a resolution bump, it's hard to see Apple going to ~400ppi on a 9.7" display. Maybe they'll surprise us and use 2304x3072 on both the Air2 and the Pro.
Or Apple could decide to seize the day and release a 4K iPad Pro:
iPad Pro 12.9" (4X): 3072x4096, 397 ppi. (f ck yeah)