I'm not *sure* it is technically possible on Lightning - with Lightning, the video adapters are tiny ARM computers that pass the display data from the iPad to the HDMI output. But with USB-C, I would think video would again be a native part of the iPad hardware- such that say, that Apple USB-C adapter with the HDMI and USB-A port would work. If you connect lightning to a USB hub, I don't think there is any way to then get video output. Not to say they couldn't engineer a Lightning 2.0 or whatever to do that. But then why do that when the work is already done (more or less) with USB-C... Really hoping!
And then obviously, if you have your iPad connected to a 29 watt USB-C adapter through a dongle/hub... You should be able to charge your iPhone from an attached USB-A or C port from that

(one fewer outlet to hog up at the airport).
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If this iOS 12 video out is any indication though, it's simply a requirement to go to USB-C. They would have to probably engineer Lightning 2.0 in order to get 4K passed to and from it, and if they use the current method of having a tiny "video computer" inside of the adapter, I can only imagine it would be 3 times as expensive, and still have low quality and high latency like the current adapter. So it may simply be smarter for them to use USB-C since most of the engineering is already done- they already ship devices with USB-C video output. They can still control what works when connected to the USB-C port, just like they already control what works if you use a lightning to USB adapter and plug something in.