I don't want to quarrel with his sense of disappointment, but it might be fun to consider: forget about race. What about the question whether we should extend the vote to people generally, including women, those without property, the great mass of (e.g.,) illiterate urban immigrants? As Answers.com recounts:
From the early national years to the Civil War, states were free to deny the right to vote with regard to a wide range of conditions, including gender, religion, race and ethnicity, citizenship, residency, tax status, wealth, literacy, mental competence, criminal conviction, and military service.The idea of "unversal suffrage" (to put the point differently) was an idea that wasn't near to being born yet in Lincoln's time. To restrict the suffrage of blacks could easily be understood as no more than a subset of a general case.
Today we tend to look back with condescension on the idea of limiting suffrage, and with good reason: Jim Crow southerners long used phony suffrage restriction as a device to keep blacks (as one might say) in their place. And just about everybody recognizes today that a general prohibition on the the suffrage of women, was pretty much of a damn fool idea by any measure.
But the idea of some sort of restriction on suffrage--bona fide, straight-up literacy or basic citizenship tests, for example--are not in principle a bad idea. I'm not proposing that we roll back universal suffrage--the costs would clearly outweigh the benefits, and anyway, the lumpen stay away from the polls in great enough numbers substantially to mitigate the evil. I'm just sayin' a rational case could be made, consistent with an expansive view of human dignity. Were I Professor Gates' friend, I would urge him not to worry about Lincoln on suffrage: here at least, we don't need to see Lincoln's concerns as race-based at all.
Afterthought: Sounds like a good book in any event, worth looking forward to. And indeed, I'd like to more about Gates' work on black American racial ancestry, which sounds like a fascinating project in itself.