For the rest of 2018, I think BNA will get another WN city or two, possibly a new G4 addition, and I think we could see an Iceland flight announcement. I'd be very surprised if NK jumped on Nashville this year. Other than that, I think that'll be a wrap for 2018 air service at BNA.
Airlineroute just confirmed this morning the Sunday only BNA-OKC flight. That's a head scratcher to me, it could probably exist daily, but I guess they are funneling travelers via STL.
I will agree more new routes aren't likely. An add by one the the US3 maybe, A couple from WN, but that would be it. As construction ramps up it will likely remain constant, perhaps with some aircraft upgrades, such as what DL is doing to ATL with the 757's.