It will by now be fairly obvious to you that not only the most interesting but by far the most comprehensive (and catholic!) selection of possibilities has been provided to you by "soliloquy" in this thread.
Inevitably, your question here is one of those to which adequate answer/s will by definition have to be thousands of times longer than the question itself, the available repertoire choices being as vast and varied as they are within the immensely wide parameter that you set. You do not say what kind of duration you are looking for here, although I presume you are talking about something of less than 30 minutes. I should slightly amend "soliloquy"'s suggestions about Sorabji, to the extent that Sorabji wrote very few works in the 1980s, although it is true that those that he did write at that time are relatively short (by his usual standards, that is); on that tack, if you are interested in considering other Sorabji works of well under half an hour's duration, then any of those featured on Donna Amato's Altarus CD (AIR-CD-9025) - which include some of what "soliloquy" was referring to as well as other pieces well worth exploring - might fit the bill. His last major work - Passeggiata Arlecchinesca (1981-82) - is a real case of a composer going out with a bang and is well worth the effort (the manuscript is almost unreadable, but fortunately there exists a handwritten edition by Donna Amato, who gave the work its world première). Then what about Carter's Sonata or Night Fantasies? - these are regarded almost as classics of 20th century piano repertoire now - and they're each perhaps surprisingly effective and skilful in pianistic terms, given that Carter, although he played the piano, was never a "pianist" per se. Well worth going for. Carter has much more recently composed a piano piece of some 10 minutes' duration entitled "Intermittences", premièred late last year by Peter Serkin (for whom I believe it was written); I've not yet heard it, but this could be a possibility too.
Best,
Alistair