The only limitations to what goes on top of your popcorn are your imagination and what actually tastes good. Chowhounds have done lots of legwork already. Garlic butter is a favorite topping, and tossing popcorn with grated Parmesan is equally popular. Either or both are often used with favorite spices and spice combinations:
Ground coriander, salt, cayenne, and sumac
curry powder or chili powder
Old Bay seasoning
Celery salt and black pepper
Dill weed and sea salt
Ranch or Italian dressing mix
Tabasco (especially with garlic butter)
Here are some more ideas to try:
Make “pizza” popcorn: Saute minced garlic in butter; add dried oregano and paprika, mix with the popped corn, then toss with Parmesan.
Go the luxe route, like lunchbox: pop your corn in duck fat (dash of chili oil optional), and season with truffle butter and Parmesan.
La Dolce Vita tosses popped corn with kosher salt, pours on maple syrup, and sprinkles with smoked ancho chile powder, saying the smoky/spicy/salty/maple-y flavor combo is intriguing.