TYS also seems to fit the bill for ATL service. And of course there is ANC, if they are looking at expanding seasonal markets further.
It's been a slow few years for WN expansion in terms of new domestic cities - I'm guessing this new expansion trend will continue for the next few years
WN had previously stated that it was considering adding service to ANC, and there was a presentation given by Dave Harvey on behalf of Southwest Airlines at the 2016 ACI-NA JumpStart Air Service Development Conference back in June 2016 that said that ANC, CVG, HNL, OGG were the only 4 U.S. markets in the top 50 that weren't served by WN in June 2006.
There was also an article in the Dallas Business Journal titled "Where will Southwest Airlines fly next: Hawaii, South America or elsewhere?" that said that WN is already considering adding service to Alaska, and that article can be found at https://www.bizjournals.com/dallas/news/2017/01/24/where-will-southwest-airlines-fly-next.html
While WN currently serves only ATL nonstop from RIC, WN adding RIC-MDW and RIC-DEN nonstop service are possibilities with
(a) Chicago and Denver being two of the top destinations traveled to from RIC that WN doesn't currently serve nonstop from RIC,
(b) MDW and DEN both being major focus cities for WN,
(c) connecting opportunities available at MDW and DEN that aren't available at ATL on WN,
(d) WN having recently added nonstop service to DEN from a few other destinations inherited through the WN-FL merger such as CLT, DSM, MEM, and ICT, and
(e) a significant amount of passengers traveling to Chicago, Denver, and points further west from RIC prior to the COVID-19 pandemic.
WN adding service to GSO, FAT, and TYS are also possibilities with GSO, FAT, and TYS being located in three of the largest U.S. metropolitan areas that aren't currently served by WN.