Soy sauce, chili sauce, chili crisps, rice vinegar, curry powder, or any combination of them. Adding scallions or grapes is also good (maybe not both, never tried it).
Lemon juice, pepper, and parmesan is also good.
Both of the above assume some mayo is used as a base binder.
Basically fat, acid, salt, and heat (in this case spice, but you can also add bread crumbs and egg to make tuna cakes)