There is a truly legitimate possibility that the hollow standing before you in indeed Ichigo. Zero nailed it I think. Paulbee alluded to it, by stating "but, she's not preggers yet." Ichigo could have been an extremely powerful vasto-lorde. Cleansed by the power of Quincy and Shinigami, he reverts back to a human form. Isshin and Masaki adopt this young boy who has no memory of his time as a VL. I really hope it's not, 'hollowfication as hiv-transferred at birth to ichigo.' Because do you know how entirely LAME that sounds?
But it would make no sense then that this hollow turned back human would then possess shinigami and quincy capabilities, not to mention a zanpakuto related to Isshin's own. I think in this case the KISS rule/Occum's Razor is applicable.