This does bring up an interesting point. What if Ichigo's mom was a hollow in a gigai that somehow defected so-to-speak? Additionally, what if Ichigo's dad was a former shinigami? That could lend more information as to how and why Urahara was able to make a shingami/hollow hybrid. Just a thought; although I am thinking that the Grand Fisher killed Ichigo's mom and he'll somehow meet up with him later to finish off the job.