This may come across the wrong way and I realize there's likely no benefit to saying it but:
These aren't undiscovered gems, they are nice places to live and have always been that way. They are places that used to be called flyover, backwoods, etc. They are nice because they were ignored. They are liked because they don't have the problems of the places people are flocking from. It seems that the coasts have found out the interior isn't all outhouses and dirt roads. It isn't that it is a discovery, it is knowledge gained of an existing fact. /rant (but not really apparently)
Arkansas is a great example. It is a trope to make fun of it and look down on the state when, mostly, it is absolutely gorgeous. There's some flood lands on the east but it is called the Natural state for a reason. Northern Mississippi is rolling hills and Oxford is beautiful. Many of the gems you're looking to learn about are likely too small to make this list, or are a combination of smaller towns making a slightly larger area. Then you have these historically depressed, shunned states being piled on because they are now seen as gems, it is a power imbalance that feels a little intentional.
You mentioned Chattanooga so you'd probably be interested in
Savannah (GA or TN)
Oxford MS (4 of the biggest, best fishing lakes right there)
Anywhere in Maury / Marshall County TN
Anywhere in middle TN, especially Cooke/Crossville (TN tech is there) except for the places already "discovered"
Knoxville / Maryville
Johnson City / TriCities TN / Bristol VA
Asheville / Boone, NC
Huntsville AL (they have rockets!)
Surrounding LittleRock AR (or for small town Heber Springs, Mountain View)
I feel old and fuddy-duddy now, get off my lawn and all of that stuff. I really like new people and new influences, I'm glad others appreciate what I have for so long, it just hurts a bit to have the continual view of condescension that comes from things like "found, discovered, etc" it implies that it needed approval in the first place when, in fact, it has been fine all along.
This was way too-ranty and possibly exclusionary, I absolutely love Tennessee and most of the South in general (looking at you Kentucky), probably to a fault. I want my kids to grow up and have access to all that I did. Land is being bought in droves, it is leaving, it is noticeable. Access is being restricted. There's a lot of great conservation work and, admittedly a lot of newcomers who help more than existing. It just feels like a bit of a race at times.