My money is still on the lineup by the end of the year being two economy models, and two pro models.
iPad Models: 2048 by 1536 pixel resolution
iPad Mini - 7.9" 326 ppi
iPad - 9.7" 264 ppi
iPad Pro Models: 2732 by 2048 pixel resolution
iPad Pro 10.5" - 10.5" 326 ppi
iPad Pro 12.9" - 12.9 264 ppi
iPad Pro will be distinguished by having features above and beyond the base consumer iPad, similar to the MacBook line. The purpose of two lines is to have baseline models which can compete with Amazon/Samsung while still having a product which can receive new/premium technology (and have a higher margin).
The two lines obviously are not geared like the classic iPhone model, where last year's premium model is this year's discount model. Instead, the the baseline iPad models get revised for new parts which can fit into their price profile, similar to how the iPod Touch worked.
Example differentiating features based on current lineup:
- Higher quality display
- True-tone
- P3 wide gamut
- Fully laminated display
- Better CPU / GPU
- More integrated RAM
- Higher core count
- Possible TouchID-integrated display
- Apple Pencil support
- Smart Connector support
- Possibly higher quality speakers
- Higher Flash storage capacity
- Fast charging power supply
And some more long shot ideas:
- Possibly exclusive cellular connectivity (or cell capability becomes custom order on non-pro models)
- Thunderbolt 3
- Integrated payment support for cash register, etc applications (and building on top of p2p Apple Pay)
With this, the reason for the iPad delay could possibly be explained by targeting a lower price for the consumer iPad line than the current hardware can target. Possible evidence of that is getting the 9.7" iPad refresh this year in time for EDU evaluation/purchasing - containing some revised parts, a cheaper screen technology, and released at a significantly lower price. My abet is that Apple still has a lower target price for the 9.7" iPad in mind to compete in the EDU market against chrome books.