There was a Martin Luther King Jr. Google Doodle yesterday in the States.
A hero in a turbulent era of history, Dr. King's life makes obvious subject matter for a time-travelling series, albeit maybe too obvious and potentially not much fun. Political people are sidelined compared to artists (who meet monsters like their own work) and scientists because their life stories are more serious by default. King's most significant enemy was J. Edgar Hoover.
So maybe the travellers meet him in passing, or have a chance to hear the Dream speech for themselves, on the way to another adventure, maybe one where they take on oppressors and help those in need.
"We must use time creatively."