Witbier, also known as a Belgian Wit, is a good style to add other ingredients to, but has a limited selection of possible additions.
Tree-fruit ingredients pair well, like apple, pear, apricot, and peach. Other fruit flavors like blueberry, raspberry, blackberry, grapefruit, watermelon, strawberry, orange peel, and lemon peel pair well. A citrus flavor like coriander likely works in a Witbier too.
A Witbier is a potential base style for a tart ingredient. Ingredients like lemon grass, hibiscus, lemon pepper, and rhubarb would be a possibility.
Some unexpected ingredients including mint, anise, and ginger maybe would work with the spicy flavor of a Witbier. A nutty adjunct including pecan may work as well.
Avoid smoky and boozy flavors like chicory or bourbon- these don’t pair well with the yeast characteristic of the style.
BJCP Style: 24A