On the two hundredth anniversary of the birth of Frederick Douglass, here's a challenge: is there any way he could have become President of the United States? (Yes, I know that in 1872 in OTL he was nominated as "Victoria Woodhull's running mate on the Equal Rights Party ticket. He was nominated without his knowledge. Douglass neither campaigned for the ticket nor acknowledged that he had been nominated."
https://en.wikipedia.org/wiki/Frederick_Douglass In any event, the ticket obviously stood no chance of winning.)
It seems overwhelmingly unlikely, give the prevalance of racism--but here is what I consider, if not a completely plausible scenario, in any event the least implausible:
After the successful (in this ATL) annexation of Santo Domingo, the state (as it eventually becomes) elects Douglass (who becomes a nominal resident of it even though he in fact lives in Anacostia most of the year) to the Senate as a reward for his supporting annexation.
https://books.google.com/books?id=KXgrCH0bkHwC&pg=PA88 A Republican Senate eventually elects Douglass (as a reward for his party loyalty, and to appeal to African American voters) to the "mostly symbolic" office of President Pro Tempore. Then all you need (before the 1886 revised Presidential Succession Act) is a double vacancy in POTUS and VPOTUS. And indeed in 1881-5 there was no vice-president--and a president who had been diagnosed with Bright's Disease--which, let us say, in this ATL kills him sooner and more suddenly than anyone had expected...