I think it would be easier to offer concrete suggestions about building up a repertoire if your reasons and purposes for doing so were known. The various motivations and goals of students, professionals and amateurs tend to differ greatly, as do our skill levels and musical interests.As an amateur playing for my own enjoyment, for example, there's no real need for me build a repertoire of breadth and diversity. My practice routine consists of learning the repertoire that interests me the most by addressing its technical challenges; I feel no pressure to master a piece to performance standards or even to memorize. Those who do, on the other hand, would probably structure their sessions differently and incorporate playing through learned pieces periodically in order to retain them at a high level.Are you ultimately seeking advice about developing a personal "library" of learned music that you can recall easily if you wish to (or need to) play on short notice? Or are you looking for suggestions concerning the specific pieces, periods and composers that comprise a typically well-rounded repertoire?
What stuff do you have right now?Try finding some time periods that you have the least stuff; if your lacking in Baroque (alot of people play a two part invention for a baroque piece for an exam, and don't like playing any more than that), look at Bach's Sinfonias, or if you are at the level to play Preludes and Fugues.A few shorter works from a set is good to get some varying repertoire, rather than tackling a 15 minute piece. A few Rachmaninoff Preludes, Debussy Preludes, Chopin Waltzes, Beethoven Bagatelles, Scarlatti Sonatas, etc etc.