1080p
1080p should be the minimum of all smartphone displays. In fact, at 440 ppi, viewing it at over 12 inches viewing distance (way over normal holding distance), you can still see the individual dots. So 5 inches at 440 ppi is great but it offers room for more improvement.
With normal 7-8 inches viewing distance, at 900 ppi, you can just barely see the dots. And guess what? 880ppi, is double 440ppi of the 5 inch. And with 4k just around the corner... You can have 4k in 5 inches using 880 ppi, and that would be perfect for a display in the future to match the 4k television sets and movies coming out.
But lets say technology is not catching up fast enough. Then we can use the current 440 ppi to make a 10 inch tablet display. It would fit into 10 inches diagonal, perfect for a tablet computer with 4k resolution.
Current monitors and TV's are selling full HD minimum resolution. Movies are being released at 1080p minimum. In 3 years time people will start moving to 4k resolution televisions. To meet technological progress, smartphones should be 1080p at 5 inches diagonal now! And a tablet should be 4k resolution at 10 inch diagonal in 3 years time. These are the resolutions that all companies that make displays should aim for.
With paper printers making 1400+ dpi common, stopping at 326 ppi of the retina display is slowing progress. dpi and ppi are the SAME thing. In fact, many monitors manufactures will substitute dpi instead of ppi. Pixel is dot. Dot is pixel. The iPhone is last years technology if it stays with 326ppi. The closer you move a display to your face, the more ppi/dpi is required. In fact the reason 1400dpi is common is that when people are examining things in detail, they will move it within 6 inches of their eyes. 1400+ is needed to so they won't see "dots". So I would say close to 1000ppi should be standard for all displays in 5 years time, and premium displays would aim for 1400+, matching whatever the printers are aiming for.
In fact, they shouldn't even be aiming for 1000ppi 2D displays. They should go full holographic 3D displays. If you have seen a holograph sticker, you can move left and right and it should rotate with your eyesight. Nintendo 3DS has this. You can even make it vertical 3D as well (so full 3D left right, up down). These displays require even more pixels. 2000ppi would allow you to get a good 3D holographic display with barely enough good size for viewing in front of you. But because technology is not moving fast enough, we probably need ANOTHER 8 years, unless someone thinks ahead and aims for it RIGHT NOW! Full 3D (up down left right) holograph color stickers were out over 20 years ago! Why are there STILL no displays able to produce full 3D without glasses? (that includes up and down 3D). The technology movers are TOO SLOW! They could land on the moon in 10 years time, but in 20 years time they STILL CAN'T replicate on display what they can on paper.
