Apple Pencil Support
The iPad Pro is currently the only iPad that fully supports the Apple Pencil, as it comes with a special display that helps the new stylus do its thing. It's rumored that the same display technology will be put into the iPad Air 3.
The report comes from 9to5Mac, which has been highly reputable with past Apple rumors. Plus it really just makes sense for Apple to do this, as expanding the usage of the Apple Pencil would be beneficial.