Orange and/or lime juice, onions, cilantro are absolutes.
I usually add a splash of tequila, and occasionally throw in some triple sec.
I've tried: chile powder in various forms, cumin, garlic, and different peppers for some spice. I still haven't found a "killer combo" yet.
LBGEx2, Santa Maria Pit, OK Joe Offset, 3 Sizes Weber kettle.