Install more faregates at the Gallery Place 9th St. entrance. Right now there are only five, and there's often at least 1 out of service.
There are often only 4 working faregates at this entrance/exit. Between the fact that one faregate is for handicapped access (ergo, slower to use) and the need to have gates for both exit & entry, this can cause significant backups. Given that this entrance serves numerous nearby uses--the Spy Museum, MLK Jr. Library, the F St. retail corridor, and nearby government and private-sector offices, among others--it is ridiculous to have so few faregates.
Also, there is plenty of room to add faregates south of the station manager booth. Right now there's a handicapped gate, a regular gate, and then a long fence running to the wall at the base of the escalators.