The phrase "existing fanbase" refers to a group of people who are already fans or supporters of something, such as a brand, musician, or TV show. Full definition
These are just my updated thoughts, because unless you're someone trying to break into the industry, I can't see this film expanding its already existing fanbase anytime soon. (rottentomatoes.com)
They also have existing fanbase among Marvel and DC comic - book readers who are perpetually excited to see their beloved superheroes (or supervillains) come alive on the big screen. (indianexpress.com)
Mean Girls and Frozen are outpacing critics» favorite The Band Visit in average sales, likely due to their brand recognition and existing fanbases. (fortune.com)