To increase volume without changing flavor:
Whip room temp frosting on medium-high for several minutes, it’ll look slightly lighter in the end.
To improve flavor:
Vanilla or Almond- add 1 tsp of vanilla or Almond extract to any flavor frosting
Orange or Lemon- add 1 tsp orange or lemon extract, or 1 Tbsp grated zest, to vanilla frosting
Cream cheese- add a package of room temp cream cheese to any flavor frosting
Marshmallow- mix in 1/2 c of marshmallow fluff to any flavor frosting
Peanut Butter- Add 1/2 c peanut butter to vanilla or chocolate frosting
Espresso- add 1 tsp espresso powder to vanilla, chocolate, or caramel frosting
Mint Chocolate- Add 1/4 tsp mint extract to chocolate icing
Adding flavor might make your frosting too thick or too thin for your liking, so to improve consistency:
To thicken frosting- add powdered sugar, 1 Tbsp at a time/ You could also use cornstarch or arrowroot powder, 1 tsp at a time.
To thin frosting- add 1 Tbsp milk or water at a time