Randal Quarles, a former Wall Street lawyer appointed to the Federal Reserve Board by President Donald Trump, is leaving the central bank. Quarles, who spent four years as head of bank supervision, announced yesterday that he will depart in December. His open seat will allow the Biden administration to reshape the central bank’s leadership. Here’s what some Fed watchers think will happen next: There will likely be more regulation of the financial sector, broadly defined. Quarles watered down some regulations enacted after the financial crisis, drawing criticism from Senator Elizabeth Warren and some other Democrats, who want tighter oversight of banks. Karen Petrou, a consultant and banking regulation expert, said the next head of supervision would likely take a closer look at Bitcoin and other nontraditional areas of finance.