While AA is currently the only airline serving CLT nonstop from SFO, UA adding SFO-CLT nonstop service is a possibility with
(a) banking industry ties between SFO and CLT,
(b) 18% of passengers traveling between SFO and CLT were connecting on airlines other than AA in Q2 2019,
(c) UA FF's in the San Francisco Bay Area who prefer to fly on UA over AA,
(d) carriers other than AA such as UA, WN, or AS being the airline of choice for majority of travelers in the San Francisco Bay Area,
(e) San Francisco-based Wells Fargo having a significant presence in Charlotte,
(f) CLT being the largest market in the 50 states that UA doesn't currently serve nonstop from SFO,
(g) UA being able to offer international connections to Asia, Australia, and New Zealand from CLT through SFO (and vice versa) on UA and Star Alliance Asia-Pacific partners if UA adds SFO-CLT nonstop service.
I have also previously mentioned UA also adding SFO-MKE nonstop service with the lack of daily nonstop service to MKE from the San Francisco Bay area being a huge hole with WN reducing SFO-MKE nonstop service to Saturday-only in Summer 2020. Posts regarding UA possibly adding SFO-MKE nonstop service can be found at https://www.airliners.net/forum/viewtopic.php?f=3&t=1411813&start=100#p21853265
In addition to SFO-CLT and SFO-MKE, UA could also add nonstop service to ALB, BUF, CHS, DSM, ELP, JAX, LIT, SDF, MEM, ORF, RIC, and TUL out of SFO with these destinations being top domestic destinations that UA doesn't currently serve nonstop from SFO.