Current lineup:
Mac: 8 base models
iPad: 6 base models
iPhones: 8 base models
I think an additional 12.9 Air will not mess the lineup up much. A larger non-Pro makes lots of sense for pen and paper replacement as well as medium consumption.
Ideally I like to see:
iPad Pro 12, 14 inch (these sizes nearly fits within the foot print of 2017 iPPs)
iPad 9, 11, 13 inch (basically built of the Air)
iPad SE
Differentiate between iPad and iPad Pro on SoC, camera, screen tech and SSD size. I have my personal doubts about a a mini Pro if a iPad mini exist.