I chose Athena and Dionysus because I think those are the two who are most capable of stepping into Zeus's sandals, though I think that neither one of them would be better than their father in the short run.
In brief, Athena has the power as well as the prestige, as shown by how she crushed Ares and Aphrodite in personal combat as well as how she competed with a senior member of the pantheon for the right to patronize Athens on even footing. Furthermore, she oversees war, wisdom, and justice, which are the sort of things that are appropriate for a pantheon head. Finally, while the prophecy specifies that it would be a secondborn son of Metis who would overthrow Zeus, Scion is exactly the right sort of setting for playing around with such statements, though making Athena a man strikes me as the wrong way to go about doing it.
Regardless, Dionysus is my second choice because Dionysus is a capable, charismatic figure in spite of his often ridiculous depictions in modern media, who seems to be weirdly convincing in a way that he really shouldn't be. Furthermore, if Dionysus is indeed the second incarnation of Zagreus, then an argument could be made that he would have the backing of Hades and Persephone, which would be critical because there is no way that anyone is going to be able to overthrow Zeus without a lot of coalition-building.
In contrast to these two, Ares is out because he's neither liked nor respected, Hephaestus is out because he's respected for his skill but the idea of him as a leader would probably cause the rest of the Olympians to break out into open laughter, Hermes is out because while there are tricksters in charge of other pantheons, he doesn't seem to have the sheer raw power to back up his shenanigans, and Artemis is out because she's literally a child. Out of these, Apollo is probably the best choice, but he has two problems in that, one, he's not really powerful in the same way as a pantheon head, and two, I feel that he's not satisfying as a narrative choice.
