names and honorifics
In Japan, people would call each other with their last names out of respect. For example, Kaneki calls Nishiki and Tsukiyama by their last names instead of their given names (Nishio-senpai, Tsukiyama-san). When they become closer, one would call the other person on a first-name basis, although some of them would still use the honorifics.
Kaneki has always called Touka with her first name and the -chan suffix, which is generally addressed for people with younger age or children. He probably calls Touka by following how others address her (especially Yoshimura). Touka is 3 years younger than Kaneki, so the way he uses –chan is appropriate. He also calls Hinami with “-chan” suffix as well, because she’s much younger than him.
Usually, Japanese people would further remove the honorifics and the to indicate closeness. We might expect Kaneki to call her wife without the -chan honorifics; but somehow, I really like his way of calling her Touka-chan. To me, it just sounds very adorable, and ever since Yoshimura’s gone, others don’t call her that way anymore; it makes him special that way.
On the other hand, almost everyone calls Kaneki by his last name instead of his given name—even Hide, his best friend. Yomo is the only one who addresses him by his given name, Ken.
In the beginning of the series, Touka doesn’t even want to call Kaneki by his name. When they become closer as friends, she’s finally willing to call him with his last name, without honorifics such as (-kun or –san). In the state that they are in now, naturally Touka also becomes a Kaneki through the marriage. Sometimes, there are women who refuse to take the husbands’ family name, though, like Mado Akira’s mother. But I think it would be improper for her to keep calling her husband by his last name in the future. I hope that she would call him Ken someday (I mean, even her uncle does; what else is she waiting for?)