Pretty happy with my Air 2...
I think it would have to get a resolution bump without taking a hit to the battery and likely an increase to 3-4 GB of RAM, as I'm looking to probably keep the Air 2 for another 2+ years. Having the battery life go back to what it was on the Air 1 or better would be nice, without having to make it bulkier or heavier.
3D Touch would be a boon too. I never had issues with missing Touch ID on my Air 1 or Mini 2 back in the day even though I owned a 5S and a 6 Plus, but I have (surprisingly) found myself trying to use 3D Touch in Mail or on links in web pages on my iPad for "Peek and Pop" so that's something I'd like to have.