Powell by far is the easisest POD, in my opinion but you could possibly have had Shirley Chisholm, after her strong run in '72, decide to run again in '76 taking her "Unbought and Unbossed" message across the post-watergate America. I think the nomination is hers if she wanted it getting her to win the General is another story.
Also, you could have Sen. Edward Brooke be chosen to replace Agnew by Nixon, and after his resignation becomes President in '74. Also Jesse Jackson without making those stupid "Hymietown" remarks, could have possibly won the Democratic Nomination in '88, but there is a slim chance that he could beat the Attwater machine in the general. So you see there are alot of potential candidates with various POD's but hardest thing is to create a climate where people are ok with electing a Black Pres earlier than in otl.