As I mentioned in my teaser post, I think the next team will be pre-1945. I seem to recll they used that pattern when revealing the ATG II teams. So I'm sticking with my vote for the Browns!
Sisler's .420 and .460+ OBP would be great! Howver I would be surprised if ATGIII has any .400+ hitters, it seems to skew valuations and performance too much. Too bad, there are a number of terrific cards/seasons that would be very fun to see in the game.
Since there are no official Strat-O-Matic Negro League cards, the odds of a Negro League team are...none.
You're right fluffy, the '37 White Sox offer very little aside from Appling and Bonura. However, so much of ATG is about value. Guys you would never consider might have hidden value depending on how they're priced.