I have observed that there are different type of bankai in the anme BLEACH. After much thinking, i have come up with 4 classifications of bankai. These are based on thier abilities after release. the types that i have come up with are...
1. Enlarged sword bankai: this is the type that increases the original size of the soul slayer, explosively increasing their attack power. this is most common in offensive type soul slayers
Examples: Hihiou Zabimaru (Renji's bankai), Ryuumon Hoozukimaru (Ikkaku's bankai), Konjiki Ashisogi Jizou (Mayuri"s bankai), Kokujoutengenmyouou (Komamura's bankai).
2.Area of Effect bankai: this creates a limited space around the user and the opponents, giving the advantage to the user
Example:Enma Koroogi (Tousen's bankai)
3.User fusion bankai: the user gains added or boosted abilities because of the fusion between him and his sword, it may also be considered that this is a condensation of the soul slayer's ability in a small form.
Examples: Tensa Zangetsu (Ichigo's bankai), Dai Gurei Hyorinmaru (Hitsugaya's bankai)
4.Special Type bankai
: this has all of the abilities of the above mentioned bankai. the level of power usage is dependent on the user.
Example: Byakuya's bankai- (Senbon Zakura kageyoshi-an enlaged sword bankai since it amplifies the number of blades) (Senkei Senbon Zakura kageyoshi-an AoE bankai since it creates an area where the combatants are surrounded by floating swords which has amplified attack force)
(Shuukei Hakuteiken- a user fusion bankai because it is the compressed form of Byakuya's blades in the form of a single white blade that has the highest attack power)
This theory is open for questioning for all of you BLEACH fans. i hope you understood my little theory about bankais. I'll come up with something new as I watch the series so wait for it. See ya!