It’s been announced that Original Xbox Backwards Compatibility will work from the disc, digital licenses will work, and you can system link across all three generations.
This was made apparent via a tweet that was sent out by Albert Penello, as seen below. Now that’s a pretty cool announcement.
Your original Xbox discs will work. Digital licenses will carry over. AND you can system-link play across all three generations.
— Albert Penello (@albertpenello) June 12, 2017