When TDOT shelved the northern loop several years ago, they said that they didn't know when or if it would return. IIRC
And, Shawn, you're right it would help in the Northeast quadrant (Lebanon, Mt. Juliet, Hendersonville, Portland, etc.) which IMHO is the one that needs help the most.
Unfortunately, Tennessee all too often builds partial loops. See Briley Parkway (TN 155), James Campbell Blvd. in Columbia, US 43/412 in Columbia, Mack Hatcher Parkway (TN 397, US 31/431 Truck) in Franklin, Old Hickory Blvd. (not a freeway but supposed to be a loop)