Ill probably be hated for saying this but 2gb ram should have been standard on the 6 plus...
I don't think there's a single Apple product that was blatantly missing a feature.
iPad Mini should have had Retina
iPad Air should have had Touch ID
iPhone 5 should have had a bigger screen
iPhone 5s should have had NFC to allow Payment with TouchID
iPad Mini 3 should have had iPad Air 2 specs
iPhone 6 should have had 2GB RAM
iPad Mini 4 should have had an A8X instead of a plain A8
iPad Pro should have had Force Touch
While I don't necessarily disagree with you, if you look back at everything we've gotten over the years then I'd say the iPhone 6 and iPad Air 2 are the most longterm iOS products they've ever made. (That said, I'm still upgrading from 6+ to 6S+)