Ha ha! I think I figured out something for Koujin's 'true Bankai'.

I was thinking about the properties that crystals have, and I remembered one that isn't commonly thought of. If a crystal grows where there is an impurity in the environment, the impurity will bind itself into the growing crystalline lattice, taking on specific properties (like color). Well, what happens if an impurity is introduced to a 'spirit-crystal'? What I'm thinking of is an adaptive ability that can be activated after an opponent attacks with a moderately strong or stronger attack (for example a Cero could work, but not balas). The Bankai then absorbs the ambient spirit particles left over. It changes color and gains some ability related to the opponent that used the attack. It'll probably be a passive type of ability, but possibly some kind of attack-thingy. Koujin will also have to deal with a little bit of his opponent's personality leaking over, though not enough to really be a problem.

I was also pondering on the appearance of his actual final release, and I believe I struck on something I like. Each sword looses its handle (the entire length of the weapon becomes a blade), and they float near his hands/arms. He also gains several smaller floating blades made from crystal in the shape of his unreleased form orbiting nearby his body. Probably little clumps of crystal near his arms and body as well, but mostly for effect rather than use. He will gain a pair of pauldrons and his armor pieces will become more spiky looking. The floating weapons can't move more than a few feet from him, but will allow him to attack from directions impossible if they were wielded normally, as well as attacking with several more weapons than someone with only two arms could.

This 'actual bankai' will grant him a boost to...well...everything that bakai usually gives. The final 'effective multiplier' he gains will be around 11 or 12 times his shikai (Bankai generally gives around 10x, doesn't it?), but he is forced to (able to?) release his final form separately from the bankai that he currently uses.

How's all that sound? good? OP? wimpy?