The entire last sequence is so important because it reveals a lot about how Joy and Nolan might be using time in Season 1. Specifically, the last few scenes either support or completely wreck the popular "William is The Man in Black" theory, which posits that Jimmi Simpson portrays William's first visit to the park, and Harris, based on what we've learned about his maze hunt, his last. If this line of thinking is correct, and the final scene does take place in the past, The Man in Black very well could be William, 30 years later. If the scene is in the present, however, William cannot be the Man in Black; they would have to exist in the same timeframe as two different people.
At the moment, it's hard to know for sure. We need more info. But if "The Stray" teaches you anything, it's that you need to pay extra attention to Dolores' scenes going forward -- how she interacts with people and what she sees. She not only connects many of the characters to one another, but, as the oldest model operating in Westworld, she also represents the key that could unlock the park's past and future.