Just a thought but maybe ichigo is supposed to be the the one to beat aizen (not only because kubo says ao but because of something i thought of the other day) zangetsu means moon cutter while Kyōka Suigetsu means Mirror Flower, Water Moon. now the importance of the names is kind of good foreshadowing on kubo's part in my opinion. Mirror Flower, Water Moon kid of hints at Kyōka Suigetsu abilities a bit. i thought of this before i read the artical on Kyōka Suigetsu stating "The name Kyōka Suigetsu hints at its illusionary properties; the moon reflected in water and a flower in a mirror are things that can be seen but not held. It is a Chinese saying for seeing and desiring something that is a dream that cannot be easily grasped in hand, teaching people about temptations".
now look at the name moon cutter. as it implies it can cut the moon but in ichigo's case with his first fight with aizen he was pretty much pwned but also at this point he hadn't had too much expereince with his bankai tensa zangetsu (heavenly moon cutter).
now if anyone knows he story of the baboon and the moon the baboon sank in the water trying to grab at the moon's reflection because of the illusion of the moon in the water when actually the moon is in the sky or as most people would like to say "heavens". like the moon in this story no one can grasp aizen because of his "moons" reflection. this is where ichigo comes in. as mentioned by unohana ichigo is the only shingami not to be fooled by aizens shikai but in this particular case its gonna take tensa zangetsu to beat Kyōka Suigetsu because of the simple fact that in order to cut a moon thats in the heavens you'll need a "heavenly moon cutter". but the trump is getsuga tenshou "Moon Fang Heaven-Piercer" because of ichigo's need to not see aizens shikai zangetsu's longrange capabilities are his greatest asset at this point. the moon cutter can pierce the heavens with its fang and cut the moon reflected off the water.
so in conclusion of having nothing to do this friday evening, the moon cutter shall cut the water moon.