My popcorn is full of flavors. Sometimes I'll add just one, and other times I'll add them all.
[*]A dash of pepper will really give you a much more dramatic experience. I usually use black pepper, but a dash of red cayenne pepper is also quite good.
[*]I've never tried mashed garlic like mentioned above, but I have used other forms of garlic quite a bit. It makes popcorn so much better. What I do is to use garlic salt to replace most of the regular salt. Garlic powder works as well.
[*]I always mix different types of butter: real butter, powdered butter, squeeze butter, etc. The more types you add the more complex and better the taste. Usually I have 5-8 types of butter and 1-3 types of oil in each popcorn batch. I always keep it at least 50% real butter, the rest are just for hints of other buttery flavors.
[*]Try adding dried minced onion to the butter as it melts. That'll soften up the onion and it adds a lot to the popcorn taste.
[*]Seasoning salts in limited quantities can really perk up an otherwise bland bowl of popcorn. Again, use these sparingly.