I'll guess that Ichigo took so much power from Rukia is because he is part Quincy. So to recap Ichigo will be part shinigami, hollow, fullbringer, and quincy. That's why Mayuri's research is going to play a part and that Aizen will as well because he recognized the same. It will all tie in with Isshin, his mom, Ryuuken, Kaien, Ginjo and the rest of the fullbringers, and the Shiba clan. Another of my crackpot theories is his mom was a quincy and brother to Ryuuken which makes him Ichigo's uncle and Uryu his cousin, Isshin is a banished shinigami captain or royal guard that fell in love with a quincy and thereby shaming the Shiba clan causing it to fall from nobility, Isshin originally discovered Fullbring powers in the human world, Urahara is the mastermind behind all of this, and Ichigo is a grand experiment.