Belgian Dark Strong Ale is a flexible style, with a lot of possible additions, but some really bad combinations you should stay away from. It’s malty and fruity, which provides a great foundation for a lot of additions. Plus, it’s the only style that the BJCP style guide refers to as “dangerous”.
A Belgian Dark Strong Ale is a great base style for a nutty ingredient. Adjuncts like chocolate and coffee would likely work, and pecan to a lesser extent.
Citrus ingredients like orange peel, coriander, and juniper berries are a possibility. While citrus is often paired with a stout, the I’ve often thought it a poor combination with the roastiness. Since this style is less roasty, it would likely be a better base style for orange.
Combinations of citrus and nutty ingredients would work well- chocolate orange would be a great addition for a wintertime Belgian Dark Strong Ale, or even coriander and coffee.
Berry ingredients are a possibility, like raspberry, blackberry, and strawberry. Other fruits like apricot or plum would also work. On all of these, the fruit would have to be quite restrained- present, but not the primary flavor.
Some other interesting possibilities are vanilla, bourbon, anise, coconut or elderflower.
BJCP Style: 26D