Custom belts for as long as I can remember have only allowed the use of text, numbers, and spaces. However, there are some special characters that you can use despite this prompt. Special characters on the qwerty keyboard that can be used are:
grave accent `
caret ^
underscore _
brackets [ and ]
backslash \

I believe that expanding the list of allowable special characters, or allowing all special characters, would open up more customization options for those who have earned (or bought enough qi to purchase) the title of Custom Belt.

At the very least I would like to see apostrophes allowed, as users currently don't have the ability to attribute a possessive status to their belt unless they want to use the grave accent as a replacement symbol (like I'm doing now). This would allow for titles such as "Butcher's Belt", "Demon's Belt" etc.