I did a bunch of searching but couldn't find anything conclusive. I'm trying out SlingTV but the basic package doesn't include CBS/FOX/NBC. I'm wondering if I can supplement SlingTV with a tuner, but I'm not really sure how it works. I do have a USB extender hooked up with it so it could be a USB tuner, if that would work
You'll have to wait until the 7.x update to the mi box which is expected sometime in the next couple months. It will allow a USB tuner to be used that you can hook up to an antenna. There is a list of USB tuners that will be supported somewhere online. After the update, OTA channels should be available via the live channels app and some kind of DVR functionality also.
Can't wait for Nougat. If live channels is a good interface i could just about ditch my TV controls altogether.
Tablo has announced the Tablo DROID and Tablo Tuner . They specifically mention "powerful Android TV set-top-boxes starting with the Nvidia Shield." so I don't know if the Mi Box is included. Expected 2Q 2017.
The Tablo DVR is another solution you could consider.
As of now your only option is a HDHomerun Connect or Extend. With your antenna hooked to this and it on your network you'll be able to use either the HDHomerun app or AndroidTV's Live Channels app to watch your OTA channels.