There are a lot of possibilities in Bleach, but one I am very infatuated with is Shunsui Kyoraku's Bankai. Remember in the fight with Espada Coyote Stark, Ukitake instructed Shunsui not to release his Bankai where "everyone could see it" and I, trying to put pieces together, came up with an idea on why his Bankai shouldn't be seen.
Shunsui's Shikai makes children's games real, which can be seen in his fight against Coyote Stark, and the name of certain attacks (i.e Kageoni, Takaoni, Irooni) end with "oni", translating to demon. After discovering this, I began thinking: does Shunsui's Bankai either summon some sort of demonic game that stretches out vastly (because anyone who's in the perimeter of Katen Kyokotsu's reiatsu has to play the [selected] game), which is why Ukitake instructed him not to use it, or is his Bankai so overwhelmingly powerful and ominous that it shouldn't be seen by anyone other than, how it seems, Ukitake, himself and Yamamoto?
Shunsui is one of the strongest captains of the soul society and that's why this puzzles me. Is his Bankai really that powerful that no one should see it? What do you think could be the case?