One major plot point I'd like to see explored (or at least expanded upon) is the five UA colony missions that were launched from 2104 to 2106 - namely, the Covenant, the Concordat, the Arceon, the Affiance, and the Allegiance.
Now granted, we KINDA know what happened to the Covenant, and the Concordat mission was a success, so there is comparatively little mystery there. The Arceon also successfully reached its destination, although we don't know where that was.
The Affiance and the Allegiance, conversely, were both lost, with no reason given hitherto. The Affiance was launched Trailward - the same direction as the Covenant - so at least we know that. Soooo, the Allegiance is the ship about which we know the least. I'd love it if we got some answers at some point.