Would Pompey truly be willing to marry Atia? His wives (save for Antistia and her special circumstances) seem to have come from the best of the roman aristocracy: Aemilia Scaura, Mucia Tercia, Julia Caesaris and even Cornelia Metella. Atia, despite the link to Caesar, seems a pretty minor option in comparison.
Pompeia would certainly be the more reasonable match either for Caesar or Caesar the Younger, as she was between 15 to 20 by the time of the First Triumvirate (a son of Caesar being born from his marriage to Cornelia Cinna would be perfect there).
Another fascinating alternative for a son, would be Caesar marrying Servilia Cepionis after her husband Silanus's death in 62 BC, and having a son before he departs for Gaul. Not only this Caesar Minor would have almost unmatched ancestry as descendant of the Julii Caesares and the Servili Cepionis, he would be about sixteen by the time of the Ides of March if they do happen, and would also be Brutus's half brother.
Atia will still be Caesar's niece with a patrician mother and a father from a plebeian senatorial gens. Sure her background is not the same of Julia (who descended from two patrician family) but she will be more on less on the the level of Mucia (who had from both sides of family plebeian cobsularis gentes)and sure on much higher level of Antistia (who came from a plebeian senatorial gens without high political connection).