improver -
a component that is added to something to improve it; "the addition of a bathroom was a major improvement"; "the addition of cinnamon improved the flavor"
component,
constituent,
element an artifact that is one of the individual parts of which a composite entity is made up; especially a part that can be separated from or attached to a system; "spare components for cars"; "a component or constituent element of a system"
additive something added to enhance food or gasoline or paint or medicine
afterthought an addition that was not included in the original plan; "the garage was an afterthought"