You can play Ravel’s Introduction et Allegro as a harp-and-piano duet, and Alice Giles did a wonderful recording with several interesting pieces on it. The Salzedo Sonata is a masterpiece, and the crowning jewel of that repertoire. It is very difficult, but not impossible, for the harp, anyway. It’s quite rewarding. I’m sure you’ll find more pieces in the catalogs. My program with piano was the Debussy Danses, the Salzedo Sonata, the Ravel, and the Berezowsky Concerto, with a few solos to open with.
Sophie, since harp and piano have almost the same range of notes, and neither can sustain the sound as long as can a bowed instrument, an ideal combination in my view would be harp and cello, because the cello can sustain notes in the bass clef, so useful in this combination, and can also use pizzicato for added effect when desired.