"On top of that" is a common phrase used to indicate an additional point or matter that needs to be considered. There are several synonyms for this phrase that can be used to convey the same intent. For example, phrases like "furthermore," "additionally," "moreover," and "besides" can be used as synonyms for "on top of that." Each of these phrases can be employed to introduce an extra point or fact that contributes to a conversation or argument. They can also be used to demonstrate how one idea is linked to another, emphasizing the importance of the added information as a necessary consideration.