Alexander had a son - Alexandros IV, he had a regent Filipos Arhidaios, diadochi were only satraps who were supposed to be placeholders before Alexandros II could reach a suitable age for reigning, but Alexandros IV was murdered and Filipos too, so diadochi became kings on their own. And Ptolemaios was called king in Egypt before he officialy became a king.
I believe thet true heirs of Alexander did not exist but first 3 of Ptolemaics and some of Seleucids were great kings who were perhaps good for that title.
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