The phrase "center of interest" refers to the main point of focus, attraction or attention in something. There are several synonyms that can be used instead of this phrase. Some of these include "focal point," "point of convergence," "main attraction," "heart of the matter," "core of attention," and "highlight." All of these synonyms can be used interchangeably to refer to the main topic of interest. The use of synonyms helps in making written or spoken communication more interesting and less repetitive. It also helps to convey the intended meaning more effectively.