Yeah, I agree with you. A series chip with X is designed for flagship iPads (except A7 for iPad Air, iPad mini 2) and the opposite is for non-iPad iOS devices. AX chips have more horsepower and can fully support heavy graphics. I really hope that next gen iPad Pro can have A12X ! 😛 Well, fall is coming and new hardwares are also coming.
But having A12X in 2018 may still be a unreachable goal because every flagship iPad hasn’t skipped numbers in chips yet (They are following the pattern of consecutive numbers):
2010: iPad 1, A4
2011: iPad 2, A5
2012: iPad 3, A5X; iPad 4, A6X
2013: iPad Air, iPad mini 2, A7
2014: iPad Air 2, A8X
2015: 1st gen 12.9 inch iPad Pro, A9X
2016: 9.7 inch iPad Pro, A9X
2017: 2nd gen 12.9 inch iPad Pro, 10.5 inch iPad Pro, A10X
So the next gen iPad Pro will probably have A11X chip? I’m not sure, but I hope that it will be A12X.