Shady brokers may have more to fear from the SEC in the coming year than investment banks and other large corporations.

In December, the Securities and Exchange Commission heralded what it called a “crackdown on brokers defrauding customers” when it sued a pair of New York traders for allegedly making unsuitable recommendations. It’s part of what commission Chairman Jay Clayton, a Donald Trump appointee, has said is a renewed focus on fighting fraud against retail investors, whom he calls “Mr. and Ms. 401(k).”

The SEC’s emphasis on retail fraud comes as the commission is pursuing fewer enforcement actions overall, and...