Urahara’s Bankai was thought up by artist Arrancarfighter from Deviant Art. Urahara mentions “My Bankai isn’t suited for training people or giving them strength” during episode 126.
Urahara’s Bankai is yet unknown but Arrancarfighter predicted what his Bankai would do. Urahara loath’s his Bankai, and rarely uses it because of how very dangerous it is; Not just towards his opponents, but also to himself.
His Bankai’s special ability is that the weaker/more injured Urahara becomes in battle, the more destructive force its attacks gains. It’s a sacrificial-type Bankai, to reach it’s full strength he’s forced to allow himself to get injured (it feeds off his injuries).
If Urahara were to ever be near death and use his Bankai its power would be enough to completely obliterate almost anything. At perfect health his Bankai is actually less useful than his Shikai.
Hopefully Urahara’s Bankai will be told soon, so till then we can only wait. Comment below and tell me what you think about Arrancarfighther’s theory of how Urahara’s Bankai would look like as well as the power of Urahara’s Bankai.