At first I thought I saw similarities between this and Shelley's Cerci, but I guess the themes in this play are a little different. The idea of the powerful oppressing the powerless is a similar theme but the two plays end up diverging quite significantly. The story itself involves the queen of Athens being raped by Apollo and then giving birth to a child. She then exposes the child (namely leaving it out in the wilderness to die) due to the shame of the whole situation. Years later her and her husband come to Delphi to find out how they can have children, and since they are the rulers of Athens children mean succession. As it turns out the child, Ion, was taken to Delphi and has been raised as an Ancient Greek version of an altar boy.
There is the idea of the powerful, represented by Apollo, taking advantage of the powerless, represented by Creusa, though remember that she is a member of the nobility, and this indicates certain layers of power. The idea here involves the relationship between the gods and humanity and raises the question that since the gods are divine why do they behave in such a fickle and unjust manner. This can be representative of the class system within Athenian society (and here were are not exploring the master slave relationship but the distinction between the wealthy and the poor, or the city dwellers and the farmers). Thus the question of justice does arise since Apollo, representing the wealthy, is able to commit crimes against humanity (representing the poor) and get away scott free.
Another theme that flows through here is the issue of progeny. Creusa and her husband desire a child so that Athens might have a stable government. The existence of a heir means that there will be no succession crisis (which can be turn out to be quite violent as various factions all fight for control of the kingdom) however they have remained childless. This issue of succession takes a twist when Xuthus (Creusa's husband) is told that upon leaving the temple the first person he sees will be his son, and this turns out to be Ion. This raises a few issues since Xuthus has had no child by Creusa, so it means that this child must have been had through another woman.
This causes a problem because Xuthus is not native Athenian, which means that if Ion is not the child of Creusa then he is a foreigner and for him to ascend the throne means that a foreigner will rule Athens (which is an issue that comes down even today – one of the theories as to why Kennedy was assassinated was because he was Catholic, which meant the Pope, a foreigner, was calling the shots). The problem was that secrets are unlikely to remain secrets, and the gods were never (and still are never) known to give straight answers. This means that the riddles that they spoke were oft known to be misinterpreted.
This play is not a tragedy in a sense, and probably falls more into a comic genre with the gods posing riddles that are misinterpreted, with the main actors making assumptions and getting things wrong, and then everything being sorted out at the end and everybody leaving happy. Ion, the one who never knew his mother, or father, leaves with a mother and a father, and Creusa and Xuthus, who desired a son, leave with a son. Athens is also saved, in that the succession crisis does not come about because an heir to the throne has become apparent.
The only person that escapes punishment, and in fact never makes an appearance in the play, is Apollo. Thus another theme that runs through the play is the question of the fickleness of the gods. It is intended by some characters that Apollo must be punished for his crimes, but Athena makes a grand entrance and tells everybody to stop and enjoy the revelations that have come to light. The play is opened by Hermes, who gives us the background and tells us the story that leads up to this point, but Apollo never makes an appearance. This is not surprising since the plays (particularly Euripides who loves to use the Deus Ex Machina) tend to only have the gods on stage at the beginning and the end. However the true villain of the peace never makes an appearance and skulks in the background somehow knowing his crime but never wanting to face it. Once again I guess this is a similar view of spirituality that many people have today – particularly when we deal with the fact that God seems to take a back seat when it comes to letting those who call themselves his people run riot over the earth.