I think that we will see a seventh generation iPad at some point during 2019. I see this model becoming more important to Apple because of the price of the Pro range.
The focus will still be on keeping costs down to make it attractive for the education market, so I wouldn’t expect to see significant improvements. For that reason, it’s likely that they will use a lot of existing parts.
I’m pretty sure Apple will stick with the same screen size. It keeps the costs down and differentiates it from the Pro models.
The big question is whether they move to a laminated display. The fifth and sixth generations used the Air chassis, moving to a laminated display would allow them to use the Air 2 or even the Pro 9.7 chassis. There are obvious cost savings in terms of research and development, and also tooling, in reusing an existing chassis.
The 3GB A11 would seem like the obvious choice for the processor.
If they used the Air 2 chassis then the selling points would be that it's thinner and lighter, faster, and has a better display.
Switching to the Pro 9.7 chassis would also allow them to offer better sound as it includes four speakers. I suspect that they would delete the smart connector, again, to save costs and differentiate it from the Pro line.