Sea salt and nutritional yeast are my favorite additions to flavor popcorn, but you can add any seasoning you like. Other things I like to add are yellow curry powder, garam masala, and cayenne pepper powder. Occasionally, I steal Annie's Mac N Cheese powder from a box. You'll end up with a box of pasta shells, but it's worth it.
If you are a fan of extra spicy you can pour the popcorn into a large plastic bag and sprinkle hot sauce (Tapatio, Tabasco, Sriracha, Yuzu Kosho, etc) over the popcorn. Then fill the bag with air and seal the top, shake the bag, and make sure the hot sauce coats to your satisfaction. You can add more hot sauce and repeat the shaking.
Bragg Nutritional Yeast