With Fantastic Beasts: The Secrets of Dumbledore being set a decade after the second film, in 1939, it means that the third movie could include Tom Riddle, aka Voldemort. The temporal leap between The Crimes of Grindelwald and The Secrets of Dumbledore is quite jarring. Presumably, Grindelwald has spent the past ten years amassing a following to his cause and Dumbledore has been plotting the best way to defeat Grindelwald. As for Newt, the future is murky especially since the third installment's title reveal seemed to shift focus away from the fantastic beasts altogether.
The prequel series has added a lot of new information to the canon and even broken canon at times. It all relates to one of the most interesting elements of the Fantastic Beasts movies: the connections made to the original Harry Potter films. J.K. Rowling brought back a young McGonagall and even attempted to add Nagini's origin story into the films. Whether this is a welcome addition to the series or detrimental to it is up to the court of public opinion. That being said, with Fantastic Beasts: The Secrets of Dumbledore being set 10 years in the future it brings about the possibility that young Tom Riddle could very well appear in the next movie, even if briefly.
Dumbledore first met Tom Riddle at Wool's Orphanage in 1938 while offering Tom a spot at Hogwarts. The scene is chronicled both in the 6th novel and in Harry Potter and the Half-Blood Prince. The meeting was unsettling enough for the headmaster to keep the memory for later viewing, and it could possibly be in the back of Dumbledore's mind during Fantastic Beasts 3. 1939 would mark the end of Tom Riddle's first year at Hogwarts. It's been mentioned by both Dumbledore and Voldemort that while Tom Riddle was in school, Albus kept a close watch on him. The series is set to end in 1945 after the battle between Dumbledore and Grindelwald when Tom Riddle is either 18 or 19 years old. At that point, Tom Riddle would already have 2-3 Horcruxes under his belt, the first being made when he was 16 and the other two shortly after. Since the two timelines intersect so purposefully, it's not out of the question to think that Fantastic Beasts: The Secrets of Dumbledore might slip the old antagonist back into the series.
Lord Voldemort would appear at Hogwarts if the villain is going to appear at all. Fantastic Beasts: The Crimes of Grindelwald played on fans' nostalgia by returning to the Wizarding school. A trip back to Hogwarts in the third film is a major possibility, considering the franchises' refocus on Dumbledore. There's also a treasure trove of memories about Tom Riddle that Dumbledore revealed in The Half-Blood Prince novel that never made it to the sixth Harry Potter movie. This is one of the main reasons that those who aren't fans of the books had trouble following the sixth film. It's possible that audiences could see some of these memories played out on the big screen since the timelines merge.
Hopefully, Fantastic Beasts won't repeat the same mistake by breaking canon or including too much from the Harry Potter timeline. Though, with Rowling's increasing need to call back to the original material it's plausible that some of Voldemort's story may show up in Fantastic Beasts: The Secrets of Dumbledore. It's unknown if the two timelines intersect for this very purpose, or if it just happened that way. Knowing J.K. Rowling, however, there's an undisclosed reason for everything.