F9 has recently announced plans to serve BUF, DRO, BDL, PHX, SAN, SFO, and TPA nonstop from DFW.
If F9 is successful on DFW-BUF/BDL nonstop service, NK might be able to make nonstop routes such as DFW-GSO/RIC work with both of these routes currently lacking nonstop competition along with F9 not currently serving GSO or RIC. RIC is also one of the top remaining domestic destinations traveled to from the DFW/DAL market that currently lacks nonstop competition and nonstop LCC service from DFW/DAL.
NK was much bigger than F9 at DFW prior to the COVID-19 pandemic, but F9 will be serving more destinations nonstop from DFW than NK after the recent F9 adds. However, NK has daily nonstop service from DFW to more destinations than F9 does (along with multiple daily nonstops out of DFW to a few destinations) whereas most of the F9 nonstop routes out of DFW are less than daily.
I doubt F9 will expand much at GSO, I'd be willing to bet they stay small are target the immediate leisure market and expand more in Concord. At most LAS might be worth it.
NK I think would be more likely to expand at GSO. I'd expect MIA, LAS, TPA and/or IAH would be targets. I'm hoping for LAS and IAH honestly, just to get the coverage of western and eastern connections for NK here. NK makes a lot of sense since WN seem prime to enter back.