Alexander didn’t need to marry a royal daughter. He could have married someone from a leading Macedonian family. Philip and Alexander were obviously very talented individuals but left a lot to be desired as human beings—very bloodthirsty when it served their interests. The Greeks were not taken in by the propaganda of a war of revenge because of Xerxes’ invasion (Macedonia had been on Persia’s side in 480). The Athenians generally had a poor opinion of Alexander.
Alexander died too young. Even if he had a heir at the age of 18, the heir wouldn't make it that long anyway.