There are some more routes that could be added by NK out of BNA such as BNA-BOS, BNA-ORD, BNA-DFW, BNA-DTW, BNA-IAH, and BNA-MYR once demand for domestic air travel returns with BNA being a major leisure destination.
All of these routes have major competition from legacy carriers (with the exception of MYR). Why would NK try and compete on them?
NK already has nonstop service out of BOS, ORD, DFW, DTW, and IAH to other major leisure destinations in the Southeastern U.S.
While WN already serves DAL and HOU nonstop from BNA, AA is currently the only airline serving BNA nonstop from DFW, and UA is currently the only airline serving BNA nonstop from IAH. NK can likely make BNA-DFW and BNA-IAH nonstop service work, even with WN operating BNA-DAL/HOU nonstop service, due to
(a) NK being able to offer lower fares than AA, UA, or WN,
(b) some travelers in the Dallas/Fort Worth and Houston markets that would rather fly out of DFW or IAH instead of DAL or HOU,
(c) NK already competing with both AA and WN on most of its nonstop routes out of DFW,
(d) NK already competing against both UA and WN on most of its nonstop routes out of IAH, and
(e) DFW and IAH both being two of the largest NK stations.
NK can likely make DTW-BNA nonstop service work, even with both DL and WN already serving BNA nonstop from DTW, due to (a) NK being much bigger in the DTW market than WN, (b) NK being able to undercut WN and DL on fares on the DTW-BNA route, and (c) NK being able to capture significant O&D traffic on the DTW end of the route if NK adds DTW-BNA nonstop service.