One thing I love about zanpaktou power system is how it relates to each characters personality. Your shikai represents who you are on the surface, the persona you show to the public. Where as your bankai shows who you really are inside, the truth about yourself.
-Unohana presents herself as this peaceful medic who only wants to help others so her shikai has a healing / support ability. But in actuality she is a savage killer who loves violance so her bankai is a literal pool of blood.
-Sui-Feng tries to be this cool and colective assassin, but in reallity she has quick to anger explosive personality. So her shikai is an assassination weapon while her bankai is a literal bazooka.
-Kyoraku at first glance is a carefree bafoon who is never serious, so his shikai has the power to bring childrens games into reality. But deep inside he is a tragic person who suffered a lot in life so his bankai is a theater of despair.
-Yamamoto's shikai are the flames of justice used to imprison or turn to ash those who go against Soul Society's law. It represents his idealistic side. His bankai represents his pragmatic side of a man who uses any means necessary in order to win (weaponizing corpses, huge range of attack that can also hurt his allies and subordinates).
-Gin plays the role of a heartless psychopath who kills for fun and is only loyal to Aizen, so his shikai is a sniping tool. In reality he had a secret ambition to kill Aizen for the sake of his loved one and his bankai is manifestation of that (God-killing Spear). It can also momentarily turn to dust which is a connenction to Matsumoto.
Those are the most prominent examples, there are probably more that I didn't realized or forgot.