If you're ever serious about becoming a professional concert pianist, the one thing that matters most after your ability is your repertoire. Consider you are a very talented pianist with only a handful of pieces memorized and an aspiration to play concerts. Unfortunately you will go nowhere.. You see there is a business side to music (just like everything else), and those people risking their money and reputation by letting you play at their venues will demand a lot from you. Of course you must be well aware that the competition is sky high and that means you will have to put more effort than to be just a good pianist.
Repertoire is mainly what distinguishes the pianists from others. One pianist can be very convincing in performing the works of one or several composers while another be completely mediocre with them and only play the works of other composers. Wilhelm Kempff was a genius interpreter of Beethoven. Glenn Gould was particularly noted for his tonal clarity that would blossom in the works of Bach. Vladimir Horowitz and Artur Rubinstein were both hailed as great interpreters of Romantic Era music. Of course they did play music outside of their comfort zone occasionally but it would not compare to their renditions of the works they were particularly famous for.
Learning new music everyday is key to expanding your repertoire. A concert pianist is expected to have at least a minimum of 2 hours recital music plus several concerti at hand ready for performance opportunities. Try many different composers first and see whose music makes sense to you. It's a matter of personal opinion, really, and it's subject to change with musical experience and time. Then when you find you have a knack for a certain composer or a certain Era start focusing on their repertoire because this is the only chance to sound your best. Of course it is needless to say that you must know a handful of works by other composers who aren't in your comfort zone. After building your repertoire you will need to maintain it regularly or all your efforts will be lost. A reliable memory is all what you will need in that stage, which is not as easy as it sounds.